Problem: GR Uninstall & GR Start lock each other

Description:
Try executing UNINSTALL PLUGIN command while START G.R. is in progress.
1. Start command takes LOCK_group_replication_handler
2. Uninstall starts and locks SQL Queries - function "open_ltable()"
3. Uninstall waits for LOCK_group_replication_handler
4. Start command tries to set the server read only mode that waits on 2)

Resolution:
Group_replication_handler class has been removed along with
lock LOCK_group_replication_handler.
With use of my_plugin_lock_by_name, server will have better visibility over
GR operations and take decisions accordingly.