Merging async-call branch changes to HEAD:

Merging async-call branch changes to HEAD:

Async-call work replaces event-based asynchronous calls with
stand-alone implementation. The common async call API allows Squid
core do call, debug, and troubleshoot all callback handlers in a
uniform way.

An async "job" API is introduced to manage independent logical threads
or work such as protocol transaction handlers on client, server, and
ICAP sides. These jobs should communicate with each other using async
calls to minimize dependencies and avoid reentrant callback loops.

These changes will eventually improve overall code quality, debugging
quality, and Squid robustness.

Most of the changes in the ICAP/ICAP* files are related to removing handler
wrappers and using CommCalls. AsyncJob is now a [virtual] base class for
ICAPInitiator. All ICAP initiators and ICAP transactions are now "jobs".