SERVER-14561 Add validateConfigForHeartbeatReconfig free function.

This function is analagous to validateConfigForInitiate and others in the same file,
and provides the validation required for heartbeat-prompted reconfigurations.
SERVER-14561 Remove ReplicationExecutor::CallbackData argument from methods that aren't callbacks.

TopologyCoordinator::updateConfig and ReplicationCoordinatorImpl::_setCurrentRSConfig_inlock are
not scheduled direclty as callbacks, and so do not need a cbData argument.
SERVER-14561 Add OperationContext* member to ReplicationExecutor::CallbackData.

This member, called txn, is non-NULL if the operation was scheduled using
scheduleWorkWithGlobalExclusiveLock, and provides a pointer to the lock-owning
context.
SERVER-14455 Implement processReplSetElect in ReplicationCoordinatorImpl
SERVER-7596 Native SCRAM-SHA-1 server side support
Merge remote-tracking branch 'upstream/master'