* Document the connector API method is_transaction_active()
* Move mysql::connection_base::is_transaction_active() to the other transaction-handling methods.
* Add more tests for mysql::connection::is_transaction_active()
* Add postgresql::connection_base::is_transaction_active()
* Add tests for postgresql::connection_base::is_transaction_active()
* Change the type of the SQLite3 transaction status from transaction_status_type to a boolean flag.
* Add sqlite3::connection_base::is_transaction_active()
* Add tests for sqlite3::connection_base::is_transaction_active()
* When closing a transaction do it in the following order: report (if any), execute SQL command, set transaction active flag to false.
Removed timezone handling in the process (needs to be documented).
Note: on_conflict does not check for constraints (needs to be documented).
Note: some of the constraints tests seem to be aiming for a different static_assert.
