The largest_key API does not support sessions that use a read_timestamp. In the MongoDB code, we have to do a bit of a dance to avoid deadlocks that result from using two WT_SESSIONs on the same thread.
If the largest_key instead ignored the read_timestamp on the session, which was the original request in
WT-7992, that would simplify the MongoDB code in multiple places. We could also use the API in more places, like getLatestOplogTimestamp().