transport: Fix deadlock in client keepalive.

When gRPC keepalives are enabled (which isn't the case by default at
this time) and PermitWithoutStream is false (the default), the client
can deadlock when transitioning between having no active stream and
having one active stream.  Subsequent attempts to create a new stream
or to close the client will hang on the transport's mutex, while the
keepalive goroutine is waiting indefinitely on a channel while holding
the transport's mutex.

This fixes #1459.

transport/http2_client.go

@@ -1298,9 +1298,9 @@ func (t *http2Client) keepalive() {
 			// Check if keepalive should go dormant.
 			t.mu.Lock()
 			if len(t.activeStreams) < 1 && !t.kp.PermitWithoutStream {
+				t.mu.Unlock()
 				// Make awakenKeepalive writable.
 				<-t.awakenKeepalive
-				t.mu.Unlock()
 				select {
 				case <-t.awakenKeepalive:
 					// If the control gets here a ping has been sent