----------------
Features:
- A queue is now also a context manager, providing transactional
semantics.
- A queues now returns task objects which provide metadata and allows
reading and writing task data.
Improvements:
- The same connection pool can now be used with different queues.
Bugs:
- The `Literal` string wrapper did not work correctly with `psycopg2`.
- The transaction manager now correctly returns connections to the
pool.