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.

The code is still more complex than I want it to be. However, these are
low-level interfaces that most developers will not have to deal with and
the complexity will be significantly reduced if we get rid of old style
function-pointer based callbacks, which is probably desirable for several
reasons. Other simplifications will be found in time as well.

TODO: src/ICAP/AsyncJob needs to be moved to src/. It is not moved yet to
preserve commit logs.