WL#10632: MySQL GCS: Update XCom in Group Replication

Since MySQL-GCS development has migrated to mysql main trees, XCom has been 
adapted to also live in the server repository according to server rules of:
- Formatting;
- Types;
- Removal of external references.

The goal of this WL is to bring latest XCom code to the latest version of Group 
Replication.

FR-1: This WL should not bring any functionality change in Group Replication 
behaviour
FR-2: This WL should not break any existing functionality in Group Replication 
behaviour