- continue with [ticket:2907] and further clean up how we set up

- continue with [ticket:2907] and further clean up how we set up
_reset_agent, so that it's local to the various begin_impl(),
rollback_impl(), etc.  this allows setting/resetting of the flag
to be symmetric.
- don't set _reset_agent if it's not None, don't unset it if it isn't
our own transaction.
- make sure we clean it out in close().
- basically, we're dealing here with pools using "threadlocal" that have a
counter, other various mismatches that the tests bring up
- test for recover() now has to invalidate() the previous connection,
because closing it actually rolls it back (e.g. this test was relying
on the broken behavior).