Memcached::setMulti() should use the setmulti functionality of memcached library

[panvid]

Right now (since over 11 years) the Memcached::setMulti() function only sets entry by entry into the memCached, with memcached::set() function. This should be fixed because of performance and time costs reasons.

[m6w6]

My memory might be rusty, but there's no such functionality in the library AFAIR.