Allow setting SocketEngineSpec.extraHeaders after init.
Deprecate SocketEngineSpec.websocket in favor of just using the SocketEngineSpec.polling property.
Enable bitcode for most platforms.
Fix #882. This adds a new method SocketManger.removeSocket(_:) that should be called if when you no longer wish to use a socket again.
This will cause the engine to no longer keep a strong reference to the socket and no longer track it.