add __del__ everywhere

Pretty much every object in `alchimia.engine` manages some state:

- TwistedEngine has a worker for connect & execute
- TwistedConnection has a worker where all its SQL is executed
- TwistedTransaction holds transaction state - this one we *might* not need to deal with since it's bound to the connection
- TwistedResultProxy holds a result proxy.

TwistedEngine and TwistedConnection are particularly problematic because the worker thread created behind the scenes is its own strong-reference root and will never be cleaned up during the lifetime of the process if these objects do not shut them down.

As seen in #30 , leaking these types of resources can give one the impression that the whole system is just freezing for no reason.