Test if using a NullPool for db connections helps db issues.

During cursory testing, using a NullPool seems to resolve some issues when multiple commands are sent to a daemon in quick succession. Specifically `(ProgrammingError) Cannot operate on a closed database.` I'm not sure if this will cause side effects though, if people could do some tests on this branch with their setups it would be much appreciated.

There is a possibility that this will also help with `database is locked` errors, but I have difficulty reproducing those to test.