(dev/core#217) CRM_Utils_Cache_Redis::connect() - Allow pooling connections

(dev/core#217) CRM_Utils_Cache_Redis::connect() - Allow pooling connections

Before
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* Every time you create an instance of `CRM_Utils_Cache_Redis`, it makes
  a new instance of the driver class `Redis`.

After
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* Every time you create an instance of `CRM_Utils_Cache_Redis`, it uses
  a shared instance of the driver class `Redis` (assuming the host/port
  are the same).
* If you're writing a driver somewhere else (e.g. the upcomgin
  `CRM_Core_PrevNextCache_Redis`), you can use the shared instance
  of `Redis`.