When a existing GTID_NEXT transaction gets a conflicting GTID generated by
server, the Group Replication plugin will generate a assert due having two
transactions with same gtid.

On Group Replication the gtid is generated after conflict detection, later than
normal replication.

Some conditions were relaxed to allow only be called when commit is done and
add message to alert when gtid is already used.

The check for macro HAVE_REPLICATION was removed due to being now correctly
defined on cmake.