Add static method to get RedisStore instances by cache name

[vdzhuvinov]

This will allow us to get the Redis store key count efficiently, because currently the size method of CacheStores is not used by Infinispan; the process key iteration routine gets called instead, which is killing performance. A while ago I had a chat with William Burns in the forum, and he mentioned this may get revised in a future version of Infinispan. In the meantime the static method can help us work around this.

public static Map<String,RedisStore> getInstances()

I'll try to get a patch for that.