ZF-5702: Memcached backend performs set() calls unecessarily
In the memcached backend's save() call, a set() is performed even after it is ensured that the key does not exist. set() is rather slow in memcache, and calling it when an add() will do is only a drag.
Additionally, save() performs a replace() when it's intending to do a set(), and replace() apparently doesn't work on older (<= 1.2.3) versions of memcached.
The attached patch resolves the issue.