objectcache: fix failing tests for non-HashBagOStuff backends

objectcache: fix failing tests for non-HashBagOStuff backends

Use real time for testing absolute expirations with changeTTL().
Otherwise, backends like memcached or redis will fail since
they do not use the mock time.

Also:
* Make SqlBagOStuff actually override changeTTLMulti() by
  using the right method name
* Check TTL_INDEFINITE more explicitly for clarity
* Rename TTL conversion methods for clarity
* Use isRelativeExpiration() in MemcachedBagOStuff

Change-Id: I9365ceb31d4e7bef65906363d42b8c3020a66346