Add an API to get the number of keys in the InMemoryCache

Hello @ArtemTitoulenko, 

Please review the following commits I made in branch 'xiao-add-key-count'.

509fe14c53a82b84eb58e0228fd2510983300d85 (2013-08-07 13:18:21 -0700)
Add an API to get the number of keys in the InMemoryCache


R=@ArtemTitoulenko