rdbms: only mention write queries in "writes pending" warnings

In LoadBalancer::runPrimaryTransactionIdleCallbacks(), when
emitting log entries about pending writes, do not mention the
callback callers, since they are not part of writesPending().

Previously, callers of getScopedLockAndFlush() would end up
mentioned in the "found writes pending" log entries triggered
by dropped connections.

Bug: T368289
Change-Id: I0c612586aa774b500bd28e2c218144b03f1be809