improve feature flag caching

 * properly cache nil per context
 * cache the full lookup in-proc only

test plan:
 * quick regression test of feature flags

Change-Id: I0d2435777bcc82d01301af96df81f179366b2fdc
treesame commit of origin/stable/2013-12-28

Treesame-Commit-Id: 11028c275ff546394dcabf846dcfe66c2aa885b9