s3:vfs_aio_pthread: make use of pthreadpool_tevent instead of pthreadpool_pipe

s3:vfs_aio_pthread: make use of pthreadpool_tevent instead of pthreadpool_pipe

pthreadpool_tevent provides a much simpler api and avoids an extra
pipe for the completion notification.

This means we now have just one thread pool, that's shared for
all async pread, pwrite, fsync and openat() calls, instead of having
an extra pool for openat() with the same possible number of threads.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>
Autobuild-User(master): Volker Lendecke <vl@samba.org>
Autobuild-Date(master): Mon Apr 23 13:30:06 CEST 2018 on sn-devel-144