Validate inserts into the oplog of a standalone for stale timestamps

It can be convenient to allow MDB nodes to allow inserts into the oplog (and is even necessary for some current restore procedures). But oplog entries being inserted with a bad (stale) timestamp trip this fassert.

The fassert is important for correctness of the typical writes replication performs against the oplog (that are still on behalf of a user operation). But there is hopefully a path for validating these timestamps.

Perhaps a strict validation that an oplog entry's ts field is larger than the current top of oplog? With an exception for an explicit `ts: Timestamp(0, 0)` as I expect the server to be choosing TopOfOplog + 1.