DATAREDIS-261 - Upgrade to commons-pool2.

Prepare issue branch.
DATAREDIS-261 - Upgrade to commons-pool2.

'JRedisPool' and 'DefaultLettucePool' have been migrated to 'commons-pool2'.

'PoolConfig' has been deprecated, and will be removed in '1.4', as all required operations are available via 'GenericObjectPoolConfig'.

Along the way a minor bug within 'LettuceConnection' has been resolved, which prevented 'RedisAsyncConnections' from being properly closed/returned to pool.

Original Pull Request: #50