Send push reply as soon as async authentication is done

This feature speeds up connection establishment in cases when async
authentication result is not ready when first push request arrives. At
the moment server sends push reply only when it receives next push
request, which comes 5 seconds later.

Implementation overview.

Add new configure option ENABLE_ASYNC_PUSH, which can be enabled if
system supports inotify.

Add inotify descriptor to an event loop. Add inotify watch for a
authentication control file. Store mapping between watch descriptor and
multi_instance in a dictionary. When file is closed, inotify fires an
event and we continue with connection establishment - call client-
connect etc and send push reply.

Inotify watch descriptor got automatically deleted after file is closed
or when file is removed. We catch that event and remove it from the
dictionary.

Feature can be tested with sample "defer" plugin.
Following lines can be added to a config file:

auth-user-pass-optional
setenv test_deferred_auth 3
plugin simple.so

It takes 3 seconds to connect with async_push and 13 without it.