Bug 4447:FwdState.cc:447 "serverConnection() == conn" assertion

Bug 4447:FwdState.cc:447 "serverConnection() == conn" assertion

After certain failures, FwdState::retryOrBail() may be called twice,
once from FwdState::unregisterdServerEnd() [called from
HttpStateData::swanSong()] and once from the FwdState's own connection
close handler. This may result in two concurrent connections to the
remote server, followed by an assertion upon a connection closure.

This patch:

 - After HttpStateData failures, instead of closing the squid-to-peer
   connection directly (and, hence, triggering closure handlers), calls
   HttpStateData::closeServer() and mustStop() for a cleaner exit with
   fewer wasteful side effects and better debugging.

 - Creates and remembers a FwdState close handler AsyncCall so that
   comm_remove_close_handler() can cancel an already scheduled callback.
   The conversion to the AsyncCall was necessary because legacy [close
   handler callbacks] cannot be canceled once scheduled.

This is a Measurement Factory project.