nsIProtocolProxyService in Firefox 3.0

 This method is an asychronous version of the resolve method.  Unlike
 resolve, this method is guaranteed not to block the calling thread
 waiting for DNS queries to complete.  This method is intended as a
 substitute for resolve when the result is not needed immediately.

 @param aURI
        The URI to test.
 @param aFlags
        A bit-wise OR of the RESOLVE_ flags defined above.  Pass 0 to
        specify the default behavior.
 @param aCallback
        The object to be notified when the result is available.

 @return An object that can be used to cancel the asychronous operation.

 If the proxy identified by aProxyInfo is unavailable for some reason,
 this method may be called to access an alternate proxy that may be used
 instead.  As a side-effect, this method may affect future result values
 from resolve/asyncResolve as well as from getFailoverForProxy.

 @param aProxyInfo
        The proxy that was unavailable.
 @param aURI
        The URI that was originally passed to resolve/asyncResolve.
 @param aReason
        The error code corresponding to the proxy failure.  This value
        may be used to tune the delay before this proxy is used again.

 @throw NS_ERROR_NOT_AVAILABLE if there is no alternate proxy available.

 This method may be called to construct a nsIProxyInfo instance from
 the given parameters.  This method may be useful in conjunction with
 nsISocketTransportService::createTransport for creating, for example,
 a SOCKS connection.

 @param aType
        The proxy type.  This is a string value that identifies the proxy
        type.  Standard values include:
          "http"    - specifies a HTTP proxy
          "socks"   - specifies a SOCKS version 5 proxy
          "socks4"  - specifies a SOCKS version 4 proxy
          "direct"  - specifies a direct connection (useful for failover)
        The type name is case-insensitive.  Other string values may be
        possible.
 @param aHost
        The proxy hostname or IP address.
 @param aPort
        The proxy port.
 @param aFlags
        Flags associated with this connection.  See nsIProxyInfo.idl
        for currently defined flags.
 @param aFailoverTimeout
        Specifies the length of time (in seconds) to ignore this proxy if
        this proxy fails.  Pass PR_UINT32_MAX to specify the default
        timeout value, causing nsIProxyInfo::failoverTimeout to be
        assigned the default value.
 @param aFailoverProxy
        Specifies the next proxy to try if this proxy fails.  This
        parameter may be null.

 This method may be used to register a proxy filter instance.  Each proxy
 filter is registered with an associated position that determines the
 order in which the filters are applied (starting from position 0).  When
 resolve/asyncResolve is called, it generates a list of proxies for the
 given URI, and then it applies the proxy filters.  The filters have the
 opportunity to modify the list of proxies.

 If two filters register for the same position, then the filters will be
 visited in the order in which they were registered.

 If the filter is already registered, then its position will be updated.

 After filters have been run, any disabled or disallowed proxies will be
 removed from the list.  A proxy is disabled if it had previously failed-
 over to another proxy (see getFailoverForProxy).  A proxy is disallowed,
 for example, if it is a HTTP proxy and the nsIProtocolHandler for the
 queried URI does not permit proxying via HTTP.

 If a nsIProtocolHandler disallows all proxying, then filters will never
 have a chance to intercept proxy requests for such URLs.

 @param aFilter
        The nsIProtocolProxyFilter instance to be registered.
 @param aPosition
        The position of the filter.

 NOTE: It is possible to construct filters that compete with one another
 in undesirable ways.  This API does not attempt to protect against such
 problems.  It is recommended that any extensions that choose to call
 this method make their position value configurable at runtime (perhaps
 via the preferences service).

 This method returns a nsIProxyInfo instance that identifies a proxy to
 be used for loading the given URI.  Otherwise, this method returns null
 indicating that a direct connection should be used.

 @param aURI
        The URI to test.
 @param aFlags
        A bit-wise OR of the RESOLVE_ flags defined above.  Pass 0 to
        specify the default behavior.

 NOTE: If this proxy is unavailable, getFailoverForProxy may be called
 to determine the correct secondary proxy to be used.

 NOTE: If the protocol handler for the given URI supports
 nsIProxiedProtocolHandler, then the nsIProxyInfo instance returned from
 resolve may be passed to the newProxiedChannel method to create a
 nsIChannel to the given URI that uses the specified proxy.

 NOTE: However, if the nsIProxyInfo type is "http", then it means that
 the given URI should be loaded using the HTTP protocol handler, which
 also supports nsIProxiedProtocolHandler.

 NOTE: If PAC is configured, and the PAC file has not yet been loaded,
 then this method will return a nsIProxyInfo instance with a type of
 "unknown" to indicate to the consumer that asyncResolve should be used
 to wait for the PAC file to finish loading.  Otherwise, the consumer
 may choose to treat the result as type "direct" if desired.

 @see nsIProxiedProtocolHandler::newProxiedChannel 

 This method may be used to unregister a proxy filter instance.  All
 filters will be automatically unregistered at XPCOM shutdown.

 @param aFilter
        The nsIProtocolProxyFilter instance to be unregistered.