Improve pipelining performance, a LOT

[helje5]

While Redi/S performances pretty well against Redis w/o pipelining, real Redis crushes Redi/S when pipelining is on.

helge@ZeaPro ~ $ redis-benchmark -P 100 -p 1337 -t SET,GET -n 500000 -q
SET: 53885.12 requests per second
GET: 58092.25 requests per second

vs

helge@ZeaPro ~ $ redis-benchmark -P 100 -t SET,GET -n 500000 -q
SET: 730994.12 requests per second
GET: 1057082.50 requests per second

Redi/S perf just improves ~20% when pipelining is enabled, while Redis does more like a ~120%.

Maybe the key here is to not process the incoming requests one-by-one, but rather in batches. (also only flush in batches).