SERVER-55030 reduced the critical section of a mutex that prevented the oldest_timestamp from advancing past the majority-commit point when starting new reads with the kMajorityCommited ReadSource (i.e. readConcern: majority).

There is a race now where the oldest timestamp can advance past the majority commit point before the read timestamp is set, resulting in a fassert.

We should use the "roundup_timestamps=(read=true)" configuration when starting these transactions and then return the actual read timestamp by querying WT.