Refactoring of the asio_link module as preparation for adding a resolver.
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