1) The asio_link object no longer directly depends on AuthSrv*; instead
the caller must provide a pair of callback objects, CheckinProvider and
DNSProvider, which are called at appropriate times to check for outstanding
configuration messages and to process a DNS message.

2) In hopes of making it less painful to write the ASIO handlers
when we add the code to send requests to upstream authoritative
servers, I rewrote the TCPServer and UDPServer classes to use
the "stackless coroutine" pattern described at:
http://blog.think-async.com/2010/03/potted-guide-to-stackless-coroutines.html

The resulting ASIO code should be functionally identical to the
previous code, but it is shorter and (IMHO) easier to read:
instead of several different asynchronous response handlers,
there's a single function for TCP and another for UDP, and the
I/O operations are all laid out in logical order.

Next step will be to move asio_link into src/lib, but I'm leaving
it here for now to make it easier to read the diff.


git-svn-id: svn://bind10.isc.org/svn/bind10/branches/trac327@2934 e5f2f494-b856-4b98-b285-d166d9295462