Made transaction.managed a no-op and deprecated it.

enter_transaction_management() was nearly always followed by managed().

In three places it wasn't, but they will all be refactored eventually.
The "forced" keyword argument avoids introducing behavior changes until
then.

This is fully backwards-compatible, except, of course, for managed
itself.
Added an API to control database-level autocommit.
Separated autocommit and isolation level handling for PostgreSQL.
Tied autocommit to transaction management.