Manages the Hibernate session for the current thread. This includes creating the session as needed, allowing the
session to checkpoint (commit the current transaction and continue) and commit the transaction automatically at the
end of the request.
Remember that in Tapestry, action requests and render requests are entirely separate, and you will see a separate
request and a separate transaction for each. Care should be taken to ensure that entity objects that are retained (in
the session, as persistent field values) between requests are handled correctly (they tend to become detached
This implementation of this service is per-thread.