The optimizations of redis storage

[onestraw]

Use SCAN for announcing and garbage collection, or whenever else it's possible. I just did some benchmarks real quick and GC times go into the seconds with a few tens of thousands of peers. This is on a non-configured redis instance, freshly installed on debian. I think I have the persistence thing on, maybe that hurts performance, who knows.

refer to https://github.com/chihaya/chihaya/pull/433

[mrd0ll4r]

SCAN for garbage collection is implemented in #472