Implement mechanism to provide non-expiring keys in KVS

This patchset implements the ability to define non-expiring keys
for dogpile.cache backends. The non-expiring keys are relevant
in the case of drivers that can automatically remove keys after
a given time (e.g. memcache). This new non-expiring-key
functionality is currently only implemented for the provided
memcached backend.

bp: dogpile-kvs-backends
Change-Id: I7e25e0049e5b8697c5cb67272b660519c3c3305e
Merge "Implement mechanism to provide non-expiring keys in KVS"