Keep track of instances of IRedisClient handed out by IRedisClientsManager.

Ported https://github.com/tvjames/servicestack-redis-pool-tracker

Simple resource tracker to keep track of instances of IRedisClient
handed out by IRedisClientsManager to help diagnose pool resource
exhaustion.
Merge pull request #198 from tvjames/master

Diagnostic tool to track IRedisClient instances handed out by IRedisClientsManager