Update MemcacheEngine.php

adding $limit=0 to clear function so it deletes all the keys for real
Update MemcacheEngine.php

removing variable assignment
Merge pull request #2852 from ujwaltrivedi/master

Update MemcacheEngine.php
Update MemcacheEngine.php

removed 0 from getExtendedStats('slabs') in my last commit, someone point that out in the comment. Adding it back.
Merge pull request #2861 from ujwaltrivedi/master

Get unlimited number of slabs when clearing cache keys in memcache.
Fix Validation::decimal() not working with localized floats.

Use similar workarounds as DboSource::value() for accepting localized
floats.

Fixes #2853
Merge branch 'master' into 2.5