Now, both initial syncer and oplog applier rely on calling oplogDiskLocRegister to update oplog visibility ts after calling multiApply. After SERVER-42925, idempotency tests also rely on oplogDiskLocRegister to mimic the server behavior. This makes it very easy for one to forget calling oplogDiskLocRegister. It seems that it is always the case that we want to update oplog visibility after oplog application. So it is probably a good idea to unify/merge the oplog visibility logic into multiApply.