Description:
------------
The new method deleteMulti[ByKey] should return only valid result codes comparable with Memcached::RES_*.
It makes it simpler to check the result and what was going on.
For now it returns an array of result codes expect a successful deletion which returns a boolean true.

Reproduce code:
---------------
$rs = $m->deleteMulti(array('key1', 'key2', 'key3'));
var_dump($rs);

Expected result:
----------------
array(
    'key1' => true,
    'key2' => 16,
    'key3' => true,
}

Actual result:
--------------
array(
    'key1' => 0,
    'key2' => 16,
    'key3' => 0,
}