Fix bug in SqliteEventEmitter.read()

Prevents exception when there are no messages in the database at the
time of calling the method.
Return deserialized message in SqliteEventEmitter.read()
Make 'location' parameter mandatory in SqliteHuey
Use multiline strings instead of implicit string concatenation in sqlite_backend
Move creation of immediate transaction into SqliteDatabase.get_connection
Remove SqliteBlockingQueue
Make sqlite_backend Python3 compatible
Make SqliteQueue.read() run in an immediate transaction again