Don't keep timeout check and ping threads running after disconnection

When the connection closes, willie.run waits config.core.delay seconds and
starts a new bot. The previous bot, however, is still alive because of the
timeout check and ping threads, and after config.core.timeout seconds, the
timeout check thread triggers a shutdown that already happened, which can
have bad side effects on modules.

New connections will start their own threads anyways.

Fixes #547
Merge pull request #549 from glandium/master

Don't keep timeout check and ping threads running after disconnection