rdbms: add ILoadBalancer::getReplicaResumePos method

rdbms: add ILoadBalancer::getReplicaResumePos method

This does what ChronologyProtector wants more rigorously and is better
named. Not all replica servers will have the same position, so they
should be compared to get the highest one.

Simplify the getMasterPos() method to only return master positions
as the other current callers do not need anything else. It will now
connect if needed as well. This should make the method naming better.

Reducing the use of replica derived replication postitions (instead
of those from the master) makes certain GTID issues less likely,
such as the matter of obsolete domain IDs.

Increase general test coverage of LoadBalancer.

Bug: T224422
Change-Id: I5420721ee339a24d09c26c38709500c7bbe797c2