Previously it used to compare between the leader and sync_standbys, while in some cases (actually most of them) the leader should be excluded.
This commit makes `matches()` method flexible:
1. The leader will be included to comparison only if requested
2. checks will be performed as case insensitive (like PG does)
Besides that, everywhere in code start using `cluster.sync.matches()` instead of `name in cluster.sync.members`.
