[MERGE] Cleanup, the Comm part

Cleaned up Comm: comm_close, comm_read_cancel, half-closed monitors,
leaks.

1) Comm_close now implements the following API:

  Comm_close does not close the descriptor but initiates the following
  closing sequence:

      1) The descriptor is placed in a "closing" state.
      2) The registered read, write, and accept callbacks (if any) are
         scheduled (in an unspecified order).
      3) The close callbacks are scheduled (in an unspecified order).
      4) A call to the internal descriptor closing handler is
         scheduled.

  Details of the above steps are being documented separately and will
  become a part of Comm API documentation.

  Since all notifications are asynchronous, it is possible for a read or
  write notification that was scheduled before comm_close was called to
  arrive at its destination after comm_close was called. Such
  notification will arrive with COMM_ERR_CLOSING flag even though that
  flag was not set at the time of the I/O (and the I/O may have been
  successful). CommIoCbParams::syncWithComm is used for this. The
  credit for this trick goes to Christos Tsantilas.

  Removed fde.flags.closing_ flag as unused.

2) Removed most of the half-closed monitoring code. Old code scheduled
   monitoring reads every main loop iteration, I think. It is possible
   that the assumption was that the handler will be activated and
   cleared once per iteration so that the new read can be scheduled. The
   design could result in conflicts between two monitoring reads and
   possibly between a monitoring read and an active read. There were
   also problems with handling closing descriptors.

   I have removed the loop, AbortChecker, and the associated splay
   tree). When user code marks the descriptor as half-closed, Comm now
   simply schedules a monitoring read callback. If the user needs to
   check whether the descriptor was marked, Comm checks whether the
   callback is present. If a user schedules a read when there is
   already a monitoring callback, the monitoring callback is removed.

   Renamed user-facing monitoring functions but left compatibility
   wrappers in place to minimize user code changes, for now.

   It is possible that the whole half-closed monitoring code will be
   eventually deleted. The above changes are meant to preserve the
   intended functionality (but without coredumps) while the decision is
   being made.

3) Removed _SQUID_LINUX_-only code that would avoid addrinfo destruction
   on connect "errors". Squid seems to be working fine without this
   code. With this code, we leak memory on many connect requests because
   of EINPROGRESS. More work is probably needed to reproduce and fix the
   true cause of the memory corruption observed earlier. Removing the
   workaround will allow us to get more bug reports if the problem is
   still there.