[PATCH] RefCount API shuffling

This polishes the RefCount API a bit.

* Shuffle RefCount.h and its unit-tests into src/base/

* Reworks struct Refcountable_ into class LockableObject in its own
header file
  + changing the reference counter accessors to a lock()/unlock() names
  + some minor symbol updates of code directly utilizing the
RefCountable_ members

With this we can begin the process of replacing our multiple different
implementations of the reference-counting pattern using LockableObject.

No code changes have been made. Just symbol polishing.
If there are no objections I will apply this in a day or two.

TODO: Followup stages which can build on this are (in no particular order):
  * update the backend of CBDATA to utilize LockableObject for reference
tracking
  * update the HttpMsg locking API to utilize LockableObject for
reference tracking
  * replace the HttpMsgPointerT locking API with RefCount<HttpMsg>
  * replace the StoreEntry locking API and move to CbcPointer

I'm sure there are others floating around as well that I have not
encountered yet. lock/unlock seems to be a popular operation in the code.

Amos