API to asynchronously sync a transaction

redbeard0531 asks:
Currently, we implement our "j:true" write concern by passing "sync=true" to begin_transaction(), which causes commit_transaction() to block until durable. Unfortunately, this doesn't allow us to do anything else on that thread while we wait. In particular, when a user specifies both "w:majority" and "j:true" it would be nice to be able to wait for both replication and journaling at the same time.

This is a reasonable request and I'll look into an API that implements this. Although not directly related it is in the same ballpark as WT-1754.