WL#8202: Group Replication: Server Interfaces (2nd push)

Affects: Server-5.7   —   Status: Complete   —   Priority: Medium

This worklog works as an umbrella of the second batch of changes
introduced in the server during the GCS project development. The work
aimed at modularization and creation of interfaces, that shall
be useful to not only to GCS but also for future developments in
Replication projects.