[PATCH] AsyncCall and CommCalls changes for cleanup-comm work.

Surprisingly small. These are the changes to src/base and CommCalls.*
where all the Call related code has been changed.

Including the generalized Subscription mechanism used by ConnAcceptor.

CommCalls.* is still a work in progress. I'm confidant that much more of
the code there can be removed as the cleanup progresses in the other
components to make them use AsyncCalls more generically.

The changes visible there are indicative of most of the changes to the
wider squid code.

Alex: removing the XXX marked const problems and testing they will work
is blocked behind some build errors from the IOCB definition change. As
soon as I have building code that wont hide const side-effects I'll be
removing that muckup.

Amos

-- 
Please be using
   Current Stable Squid 2.7.STABLE9 or 3.1.8
   Beta testers wanted for 3.2.0.2