In order to compute the all durable timestamp for ephemeralForTest, we need to keep track of active transactions in ephemeralForTest. All transactions with timestamps earlier than the all durable timestamp are committed.

With this, we should be able to implement the kAllDurableSnapshot ReadSource in ephemeralForTest's RecoveryUnit.

Note: it's possible to commit new transactions earlier than the all durable timestamp but no earlier than the oldest timestamp. With that, we should prevent the all durable timestamp from going backwards.

Note 2: if SERVER-49792 is completed before this, then this ticket should also implement the kNoOverlap ReadSource in ephemeralForTest's RecoveryUnit. The no overlap timestamp is the minimum of the all durable and last applied timestamps.