rdbms: avoid recursion in LoadBalancer when the master has non-zero load

rdbms: avoid recursion in LoadBalancer when the master has non-zero load

Add and use IDatabase::getServerConnection() method to avoid loops caused
caused by pickReaderIndex() calling getConnection() for the master server.
That lead to getReadOnlyReason() triggering pickReaderIndex() again.

Make getLaggedReplicaMode() apply when the master has non-zero load and
the replicas are all lagged.

Remove "allReplicasDownMode" in favor of checking getExistingReaderIndex()
instead. This reduces the amount of state to keep track of a bit.

Follow-up to 95e2c990940f

Bug: T226678
Bug: T226770
Change-Id: Id932c3fcc00625e3960f76d054d38d9679d25ecc