Cache abstraction layer with Redis support

[gigablah]

Changes MogileFS::Config->memcache_client to Mgd::get_cache. Cache options are now configured in mogilefsd.conf (cache_type, cache_servers and cache_ttl). Preserves functionality of the previous cache implementation (using server_settings). Adapters supplied for Memcache and Redis.

[gigablah]

And yes, I did read memcache-support.txt :p
This would be convenient for those who point, say, nginx directly to the tracker (using nginx-mogilefs-module), so it bypasses the app completely.

For Redis, device IDs are stored using sets (SADD, SMEMBERS). Socket addresses are supported (e.g. /tmp/redis.sock).

I pretty much followed the structure of Mgd::get_store.

[dormando]

Thanks a lot for doing this. I'm still trying to get enough round-tuits to properly review, but you've fixed a few good things so far. Not sure if it'll make 2.65, but if not, 2.66 for sure!

[abh]

It seems like it'd be more helpful to either default to something here or print a warning that cache_type must be specified with cache_servers.

[dormando]

Hi again! I'm a terrible person: Are you around to help deal with a few more comments on the patch series so we can toss it in?

Thanks!

[gigablah]

Sorry about that, I'll try to get to it in these couple of days.

[dormando]

awesome, thanks! Though all apologies are mine!

[dormando]

ping again! If it's fixed up within the next week it could go out in the next cut