Made the memory cache data structure derive MallocSizeOf, along with
manual size_of() implementations in malloc_size_of.

Added a Measurable struct that acts as a container for fields size_of() can be called for.

Added a new IpcReceiver used for listening to messages from the memory profiler,
and used run_with_memory reporting to register a memory reporter in the thread.
Now when a message from the memory profiler arrives, report includes sizes of public and private http caches.

Updated test file.