Syncing memcached.hhi with ext_memcached.php

[RyanGordon]

This should address #7950 by syncing the methods from ext_memcached.php to memcached.hhi

I also noticed there were also some strict-type inconsistencies in ext_memcached.php like: https://github.com/facebook/hhvm/blob/master/hphp/runtime/ext/memcached/ext_memcached.php#L341

But since this file appears to be auto-generated I didn't try to fix it there.

[fredemmott]

Sorry, assumed the spam/accidentla commennts were feedback requests.

[fredemmott]

Reverted as it caused internal test issues, will investigate and re-land, but probably not today