Fix multiprocess connection

This PR is to fix : 
1. multiprocessing issues with database connection when sending queued email
2. Fix ``UnboundLocalerror``  on ``send_queued_email`` when no queued emails are available

~~I have a hard time to test this issue of multiprocess, please advise @selwin~~

~~especially on https://github.com/SeiryuZ/django-post_office/compare/fix-multiprocess-connection#L1R57~~

~~Emails on the child process are updated correctly(``STATUS.queued`` changed into ``STATUS.sent``) but if we try to query on the parent process, it stays ``STATUS.sent`` hence providing difficulty of testing this although it run correctly on manual execution~~