bugfix: load and usage schema data race

We need to block shards to avoid schema usage by concurrent requests.
Now it can be a ping request or a watch request so it does not look
critical. We don't expect many of this requests and such requests
do not use schema at all.

Part of #218

Author: oleg-jukovec

connection.go

@@ -1368,6 +1368,9 @@ func (conn *Connection) OverrideSchema(s *Schema) {
 	if s != nil {
 		conn.mutex.Lock()
 		defer conn.mutex.Unlock()
+		conn.lockShards()
+		defer conn.unlockShards()
+
 		conn.Schema = s
 	}
 }

schema.go

@@ -329,7 +329,10 @@ func (conn *Connection) loadSchema() (err error) {
 		schema.SpacesById[index.SpaceId].Indexes[index.Name] = index
 	}
 
+	conn.lockShards()
 	conn.Schema = schema
+	conn.unlockShards()
+
 	return nil
 }