Add per-txn sync configuration support. #1074

@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.