Ignore comm_write notifications if the newly added

Ignore comm_write notifications if the newly added
ICAPXaction::ignoreLastWrite member is set. This allows kids to more safely
work around comm inability to cancel a pending write request.

When connect() times out, throw an exception instead of calling mustStop()
because we can bypass the former and not the latter. We cannot bypass mustStop
because the code may use it for legitimate stopping conditions. Eventually,
we may want to have two kinds of exceptions: bypassable and fatal.

Support icap_connect_timeout and icap_io_timeout squid.conf options.

Count I/O timeout as a service failure. This helps suspend broken services
faster and avoid HTTP processing delays when the service is optional.

Eventually, we will probably count all (or most) exceptions as service
failures.

Removed ICAPXaction members that were copied to AsyncJob some time ago.

Polished debugging.

Merged from the squid3-icap branch.