ApplierState is an enum with values Running, Draining, and Stopped. It is currently owned by the ReplicationCoordinator. Move it to OplogApplier: it belongs there, and it will be easier to unittest drain mode once the OplogApplier and ReplicationCoordinator are less tightly coupled.

Currently, the oplog application code calls ReplicationCoordinator::getApplierState to determine its state. After this step, the oplog application code will own the ApplierState, and ReplicationCoordinator will call OplogApplier methods to change the state.