refactor dns_request to use loopmgr callbacks

refactor dns_request to use loopmgr callbacks

dns_request_create() and _createraw() now take a 'loop' parameter
and run the callback event on the specified loop.

as the task manager is no longer used, it has been removed from
the dns_requestmgr structure.  the dns_resolver_taskmgr() function
is also no longer used and has been removed.