Optimize Redis usage

[mperham]

Right now the uniqueness check requires three network round trips to Redis for each job pushed:

1. watch
2. get
3. (multi / setex) || unwatch

Redis 2.6.12+ has new flags for set which potentially allow this operation with a single command:

conn.set(payload_hash, 1 || 2, nx: true, ex: expires_at)

You don't document a minimum Redis version but requiring Redis 2.6+ is your choice.

[mperham]

To be honest, I'm not positive all the semantics are met by this change but the performance win is so big, I think it's something to consider.

[mhenrixon]

Didn't really have to change anything except make it simpler. Thanks for the intel @mperham !