Handle is blocking (#12)

NOTE: This is a breaking change to the expected contract of Handler. Please update your implementation to use AsyncHandler if needs be.

We have strict ordering requirements of how we handle FileSystem requests in
LSP. As such relying on the ordering the goroutine scheduler runs requests in
leads to potential out of order mutations to the FS. As such we update the
jsonrpc2 implementation to by default block until Handle returns (note it can
still respond to the request at a later stage). For more simple use cases we
provide the AsyncHandler which will work like the previous implementation.

* Ensure handle is blocking

jsonrpc2.go

@@ -226,7 +226,10 @@ const (
 
 // Handler handles JSON-RPC requests and notifications.
 type Handler interface {
-	// Handle is called to handle a request.
+	// Handle is called to handle a request. No other requests are handled
+	// until it returns. If you do not require strict ordering behaviour
+	// of received RPCs, it is suggested to wrap your handler in
+	// AsyncHandler.
 	Handle(context.Context, *Conn, *Request)
 }
 
@@ -498,7 +501,7 @@ func (c *Conn) readMessages(ctx context.Context) {
 			if c.onRecv != nil {
 				c.onRecv(m.request, nil)
 			}
-			go c.h.Handle(ctx, c, m.request)
+			c.h.Handle(ctx, c, m.request)
 
 		case m.response != nil:
 			resp := m.response