various fixes (#1267)

* logger: remove Panic log level

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

* server/grpc: add missing Unlock in Subscribe error

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

* server: minor code change

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

* server/grpc: extend test suite with pub/sub testing

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

* server/grpc: fix invalid check and allow subscriber error to be returned

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

* server/grpc: add pubsub tests

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

* client/grpc: check for nil req/rsp

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

server/grpc/grpc.go

@@ -555,8 +555,8 @@ func (g *grpcServer) Subscribe(sb server.Subscriber) error {
 	}
 
 	g.Lock()
-
 	if _, ok = g.subscribers[sub]; ok {
+		g.Unlock()
 		return fmt.Errorf("subscriber %v already exists", sub)
 	}