* Use a non-UTC timezone when testing the PostgreSQL timezone support.
* Fix serialization of time of day values for PostgreSQL
* Fix serialization of PostgreSQL values for "timestamp with time zone" and "time with time zone" fields.
Add support for connection pooling
* Add support for connection pooling to the core code.
* Add support for PostgreSQL connection pooling with tests.
* Add support for SQLite3 connection pooling with tests.
* Add support for MySQL connection pooling with tests.
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.
