Revert "Remove unused variables in KeyValueStore.cc"

This reverts commit 23b657c704865b33aa2e0e39995712c1f583c24d.

The get_* methods have side-effects!
Revert "Remove unused variables in MemStore.cc"

This reverts commit 307ba481a2705338560ecb557741d459706bdeb6.

The get_* methods have side effects!

Signed-off-by: Sage Weil <sage@inktank.com>