ruby: get rid of RefCnt and Allocator stuff use base/refcnt.hh

This was somewhat tricky because the RefCnt API was somewhat odd.  The
biggest confusion was that the the RefCnt object's constructor that
took a TYPE& cloned the object.  I created an explicit virtual clone()
function for things that took advantage of this version of the
constructor.  I was conservative and used clone() when I was in doubt
of whether or not it was necessary.  I still think that there are
probably too many instances of clone(), but hopefully not too many.

I converted several instances of const MsgPtr & to a simple MsgPtr.
If the function wants to avoid the overhead of creating another
reference, then it should just use a regular pointer instead of a ref
counting ptr.

There were a couple of instances where refcounted objects were created
on the stack.  This seems pretty dangerous since if you ever
accidentally make a reference to that object with a ref counting
pointer, bad things are bound to happen.

All regressions pass