Add an API to allow read timestamp to round up to oldest

Applications (including MongoDB) currently have to deal with a race, where they calculate a valid timestamp for a call to WT_SESSION::begin_transaction, but by the time WiredTiger goes to use the timestamp, another thread has updated oldest_timestamp to a more recent value.

It would simplify application code if there were a flag to WT_SESSION::begin_transaction that caused the transaction to use the newer oldest_timestamp in this case rather than failing.

Something like session->begin_transaction(session, "read_timestamp=100,round_to_oldest=true");