michaelcahill Please review these changes for per-operation sync needed for the LevelDB work. I also added a piece to one of the Python txn tests.

• One minor naming point - I kept the config name as transaction_sync. Pro: It is consistently named with the wiredtiger_open synchronization value. Con: It feels long and redundant on the begin_transaction API.
• This option could have gone on the commit_transaction API instead of begin_transaction. It seemed a toss-up so I chose begin_transaction because there are already a bunch of configuration items for the user to consider.