Clean shutdown of secondaries should occur in between oplog batches, not during

Starting in MongoDB version 3.2, replica set secondaries write entries to the oplog concurrently with applying the changes described by those oplog entries to the data files. Because of this change, starting in MongoDB 3.2, the singleton document in local.replset.minvalid contains a "begin" field during periods of oplog application, indicating the newest oplog entry not in the current batch of oplog entries being applied.

In order to properly downgrade from 3.2 to 3.0, which ignores the "begin" field, 3.2 secondary nodes should only shut down cleanly at oplog batch boundaries, not in the middle of applying a batch.