Added initial shared memory cache implementation (MemStore) and integrated it.

Added initial shared memory cache implementation (MemStore) and integrated it.

Like Rock Store, shared memory cache keeps its own compact index of cached
entries using extended Ipc::StoreMap class (MemStoreMap). Like Rock Store, the
cache also struggles to keep its Root.get() results out of the store_table
except during transit.

There are several XXXs and TODOs that still need to be addressed for a more
polished implementation.

Eventually, the non-shared/local memory cache should also be implemented
using a MemStore-like class, I think. This will allow to clearly isolate
local from shared memory cache code.