Clear handlers on disconnect

Summary:
A few changes that should make our connect/disconnect more robust:

* Certificate provider handler should be set directly to and managed by the policy.
* Instantiate the socket once, synchronously on the connect method. Explicit deallocation once, synchronously on the disconnect method.
* Clear handlers on disconnect after clearing the delegate.
* Wait for the operation queue to drain before returning.

Reviewed By: passy

Differential Revision: D42664724

fbshipit-source-id: bd482acbb64a9bc9e36fb3418d4c81afa2109305

iOS/FlipperKit/FlipperPlatformWebSocket.h

@@ -23,15 +23,15 @@ NS_ASSUME_NONNULL_BEGIN
 /// A message handler used to dispatch messages received from the server.
 @property(nonatomic) facebook::flipper::SocketMessageHandler messageHandler;
 
-/// A certificate provider used to obtain the client certificate used for
-/// authentication.
-@property(nonatomic)
-    facebook::flipper::SocketCertificateProvider certificateProvider;
-
 /// Initializes an instance of FliperWebSocketTransport with an endpoint URL.
 /// @param url Endpoint URL used to establish the connection.
 - (instancetype)initWithURL:(NSURL* _Nonnull)url;
 
+/// A certificate provider used to obtain the client certificate used for
+/// authentication.
+- (void)setCertificateProvider:
+    (facebook::flipper::SocketCertificateProvider)certificateProvider;
+
 /// Connect to the endpoint.
 - (void)connect;