[SERF-211] Finalize the asynchronouse resolver API. This adds a private
interface for waking the context from a poll, so that the resolver can
signal when results are available.

* serf.h
  (serf_context_create_ex): Make the docstring a docstring.
  (serf_address_resolved_t,
   serf_address_resolve_async,
   serf_connection_created_t,
   serf_connection_create_async): Remove the experimental comments
    and fix some typos in the docstrings.

* serf_private.h
  (SERF_IO_WAKEUP_PIPE): New constant for the io baton type.
  (serf_context_t::wakeup): New member.
  (serf__context_wakeup): New prototype.

* src/context.c: Include <apr_atomic.h>
  (WAKEUP_LOOPBACK, WAKEUP_FAMILY): New constants.
  (serf__context_wakeup_t): New struct for the self-pinging wakeup socket.
  (init_wakeup, process_wakeup): New private helper functions.
  (serf__context_wakeup): Implement here.
  (serf_context_create_ex): Initialize the wakeup socket.
  (serf_event_trigger): Process the wakeup signal.

* src/resolve.c: Remove the experimental/todo top-level comment.
  (resolve): Tweak log message.
  (push_resolve_result): Wake the context when a new result is available.
  (serf__process_async_resolve_results): Return immediately if the async
   resolver was not properly initialized. Add debug logging.


git-svn-id: https://svn.apache.org/repos/asf/serf/trunk@1931128 13f79535-47bb-0310-9956-ffa450edef68