s3: smbd: Change aio_pending_size static variable to a new "aio max threads" smb...

s3: smbd: Change aio_pending_size static variable to a new "aio max threads" smb.conf parameter.

Removes accessor functions as now this parameter is set
under user control in smb.conf. Default is 100.

Note that this doesn't limit the number of outstanding
aio requests, it just causes them to go onto the
pthreadpool queue.

Now we need to prioritize pthreadpool pipe replies
ahead of incoming SMB2 requests, but that's a patch
for another day.

Based on ideas from Volker.

Signed-off-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Christof Schmitt <cs@samba.org>
Reviewed-by: Michael Adam <obnox@samba.org>