First part of fix for bug #7331 - Compound async SMB 2 requests don't work right.

First part of fix for bug #7331 - Compound async SMB 2 requests don't work right.

Gets us handling SMB2 compound async requests similar to W2K8R2
(and triggers the same client bug in the Win7 redirector). Great
thanks to Ira Cooper <samba@ira.wakeful.net> for helping with
this and to Metze for the wonderful async framework. The one
thing I need to fix to make us identical to W2K8R2 is that
when a compound request goes async at the end W2K8R2 splits
the replies up into a compound non-async reply followed by
a separate async reply. Currently we're doing the whole thing
in a compound reply.

Jeremy.