Support Redis 2.8

[ryanlecompte]

Slaves are read-only in redis 2.8. This means that the existing approach of using blpop/del/lpush to monitor a node's liveness must be changed since these are write operations. This original approach was used to avoid having to busy-wait when checking nodes, but we may just need to go back to polling given how slaves behave in 2.8.

[arohter]

https://github.com/antirez/redis/blob/unstable/redis.conf#L215

• Since Redis 2.6 by default slaves are read-only.