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.

Below you will find log messages from the async-call branch that are
relevant to the file(s) being committed.

        Removed CommDispatcher as unused.

        Use async calls for firing ready events. Deleted
        EventDispatcher as unused.

        Converted CompletionDispatcher-based SignalDispatcher into
        AsyncEngine-based SignalEngine to get rid of CompletionDispatchers.