transport: Fix deadlock in client keepalive. (grpc#1460)

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 grpc#1459.

transport/http2_client.go

@@ -435,6 +435,10 @@ func (t *http2Client) NewStream(ctx context.Context, callHdr *CallHdr) (_ *Strea
 		select {
 		case t.awakenKeepalive <- struct{}{}:
 			t.framer.writePing(false, false, [8]byte{})
+			// Fill the awakenKeepalive channel again as this channel must be
+			// kept non-writable except at the point that the keepalive()
+			// goroutine is waiting either to be awaken or shutdown.
+			t.awakenKeepalive <- struct{}{}
 		default:
 		}
 	}