Add Memcache session handler

Summary: Fixes https://github.com/facebook/hhvm/issues/3781
Fixes https://github.com/facebook/hhvm/issues/4008
Closes https://github.com/facebook/hhvm/pull/4011

Reviewed By: @fredemmott

Differential Revision: D1626210

Signature: t1:1626210:1417816002:746d11901c87dbb326d851f66952fd372a228f84