Increase expiry time to 24 hours

We're not (yet) likely to worry about changes within the space of a
day for the mirror.
Accommodate differences between redis v2.6 and v2.8

redis 2.8 responds with a -2 value for keys that don't exist however
redis 2.6 responds with a -1 value.
If the queue channel is closed, bail out

For now, fail fast when the channel is closed for our given queue
connection.