Status::Hash: Reduce redis round-trips

[pjambet]

The calls to zadd & zremrangebyscore should be consistent, that is,
we don't one want to succeed and one to fail. On top of that, we can
send both at once to prevent one roundtrip.

In set, the call to expire was unnecessary, we can use set or
setex instead, or, ideally, if we were ok with supporting only redis
2.6.12+, we could switch to set with the ex option.

Finally, in remove, we can also wrap del and zrem for the same
reasons we wrapped the two calls in create.

The use of pipelined and multi is a bit verbose, and technically
unnecessary since the ruby gem drops the multi/exec calls within a
pipelined call, and a multi call by itself uses pipelining, but doing it
as such is more explicit wrt intent:

"We want to wrap these in a transaction for the sake of atomicity, and
we might as well optimize the network calls, and let Redis be faster as
well, by pipelining the whole thing"