GOAL: separate ScanningReplicaSetMonitor specific members out from the abstract ReplicaSetMonitor class.

Background: In replica_set_monitor.h, several members are intended for unit-tests only and are only relevant to ScanningReplicaSetMonitor's implementation.

1. useDeterministicHostSelection
2. disableRefreshRetries_forTest().
   • Purpose: "speed up tests by disabling the sleep-and-retry loops and cause errors to be reported immediately"
3. areRefreshRetriesDisabledForTest()

Why are they in the abstract class right now?
dbclient_rs_test sets useDeterministicHostSelection and disableRefreshRetriesForTest but is currently expected to be ReplicaSetMonitor version agnostic.

We need to inspect the use of these test-only members in dbclient_rs_test and determine (1) if the test should remain ReplicaSetMonitor version agnostic and (2) if the members can safely be moved from the ReplicaSetMonitor to the concrete ScanningReplicaSetMonitor class.