Add --oplogMinRetentionHours argument and use it to limit oplog truncation

Allow specifying a minimum retention time for the oplog in addition to a maximum size. Oplog entries are only deleted if they both exceed the size cap and the minimum retention time.

The retention time is specified in hours, allowing fractions. The default is 0 hours (feature is off).

Use the ReplicationCoordinator::getLastAppliedOpTime() as proxy for "now" and the timestamp derived from Stone::lastRecord as the most recent timestamp in the next oplog range to truncate.