Synchronize _txData structure
Remove ScopedLock
Add transaction control mechanisms for query plans

* Add automatic session pickup from request
* Add ability to automatically commit a query plan
* Clean up transaction manager interface
* Add diagnostic route /status/tx for running transactions
* Add tests
* Add exemplary python client
* Add documentation
Add Rollback operation
Merge pull request #161 from bastih/feature/transaction_endpoints

Land transaction control mechanisms