Fix callback parameters in memcached

Summary: Pass the Memcached object to the callback function. Also fix a problem with the
HNI type annotations.

Fixes #2797

Reviewed By: @JoelMarcey

Differential Revision: D1665298

Signature: t1:1665298:1415380054:eaff8585b1327a7d8ad95655234ed0690b550656
Fix a race freeing an APCHandle

Summary: Two threads could call realDecRef at the same time, both
could see m_count == 2, both would do --m_count, but neither would
notice that it hit zero.

Reviewed By: @edwinsmith, @jdelong

Differential Revision: D1666696

Signature: t1:1666696:1415373702:cf7828aed349a11d03d031e9f443b8712b31e380