Fix Memcache return values/types

Summary: Part 1: Memcache was previously returning all values as a string, no matter what they were input as.  Now their original value is saved into the flags and restored when pulled back out from cache.
However,@fredemmott suspects this might break some internal Facebook PHP code, so someone from Facebook probably should try this out and make sure it doesn't.

This change also disables compression for integer/float values (emulating the pecl extension functionality).

Part 2: Memcache was previously always returning integers for the increment and decrement functions.  Now they return mixed/variants of int or bool (for failures).

Fixes #3789.
Fixes #3790.

Closes https://github.com/facebook/hhvm/pull/3802

Reviewed By: @ptarjan

Differential Revision: D1568454