Support expiration of backlog
Support expiration of backlog

This was previously not supported by the PostgreSQL backend, only
by the Redis backend..

For high performance, it would probably be better to support a
method where clearing backlogs and expiring could be done every
10/100 requests or 1/10 minutes, instead of doing so on every
request.

cannikin/message_bus,nathanstitt/message_bus,nathanstitt/message_bus,SamSaffron/message_bus,MadBomber/message_bus,SamSaffron/message_bus,cannikin/message_bus,nathanstitt/message_bus,MadBomber/message_bus,MadBomber/message_bus,cannikin/message_bus