Mem-cache entries exceeding one shared memory page in size, using

Mem-cache entries exceeding one shared memory page in size, using
Ipc::StoreMap that now supports multi-slice entries. For the memory cache,
each slice corresponds to a shared memory page. Added a shared memory stack of
free slices. Many aspects of the implementation are similar to Rock Store.

Do not mem-cache entries that are already mem-cached at the time of keep()
call. This change is unrelated to multi-slice caching, but may provide a
noticeable performance boost.