Use main_engine() in alembic env setup.

We enforce a few valuable options such as strict mode and the charset
with it, which are valuable to set during migrations as well.
Don't setproctitle in inbox.util.startup.

Scripts should set their proctitle to something unique themselves.
Explicitly garbage collect ScriptDirectory object in check_db().

For whatever reason, the line `script.get_current_head()` leaves a bunch
of idle db connections lying around until the ScriptDirectory object is
garbage collected. I'm not sure what it's using them all for because
ahhh source diving SQLAlchemy, but it's sure not because we're asking
for a lot of connections in our migration env config. For now, we can
work around by explicitly cleaning them up.
Always inherit from object.

See https://docs.python.org/2/glossary.html\#term-new-style-class for why.