Remove NullStoreEntry (#241)

Remove NullStoreEntry (#241)

A nil pointer is the proper way to indicate a missing heap-allocated
object in C++. Removing NullStoreEntry simplifies and optimizes code.
This removal also brings us one step closer to removing all virtual
methods from StoreEntry, further optimizing code and even saving 8 bytes
per non-shared memory cache entry on most platforms.

Also un-virtualized a few StoreEntry-only methods to optimize their
callers.