Class: PoolCounterRedis  - X-Ref

Version of PoolCounter that uses Redis

There are four main redis keys used to track each pool counter key:
- poolcounter:l-slots-*     : A list of available slot IDs for a pool.
- poolcounter:z-renewtime-* : A sorted set of (slot ID, UNIX timestamp as score)
used for tracking the next time a slot should be
released. This is -1 when a slot is created, and is
set when released (expired), locked, and unlocked.
- poolcounter:z-wait-*      : A sorted set of (slot ID, UNIX timestamp as score)
used for tracking waiting processes (and wait time).
- poolcounter:l-wakeup-*    : A list pushed to for the sake of waking up processes
when a any process in the pool finishes (lasts for 1ms).
For a given pool key, all the redis keys start off non-existing and are deleted if not
used for a while to prevent garbage from building up on the server. They are atomically
re-initialized as needed. The "z-renewtime" key is used for detecting sessions which got
slots but then disappeared. Stale entries from there have their timestamp updated and the
corresponding slots freed up. The "z-wait" key is used for detecting processes registered
as waiting but that disappeared. Stale entries from there are deleted and the corresponding
slots are freed up. The worker count is included in all the redis key names as it does not
vary within each $wgPoolCounterConf type and doing so handles configuration changes.

This class requires Redis 2.6 as it makes use Lua scripts for fast atomic operations.
Also this should be on a server plenty of RAM for the working set to avoid evictions.
Evictions could temporarily allow wait queues to double in size or temporarily cause
pools to appear as full when they are not. Using volatile-ttl and bumping memory-samples
in redis.conf can be helpful otherwise.