Add query API to get most recent checkpoint's stable timestamp

Talking with daniel.gottlieb, there are some debugging-related use cases where it would be helpful if MongoDB could query WiredTiger to know the stable timestamp where the most recent checkpoint was taken. In many ways, it is just a live version of the recovery_timestamp recently added.

I suggest query_timestamp=last_checkpoint. We can store a read-only value in the global transaction table of the same value we're storing in the table's metadata under checkpoint_timestamp similar to txn_global.recovery_timestamp.