s3/smbd: implement a serializing async copy-chunk loop

s3/smbd: implement a serializing async copy-chunk loop

Later commits will make the low level copy-chunk implementation async
using a thread pool. That means the individual chunks may be scheduled
and copied out-of-order at the low level.

According to conversation with MS Dochelp, a server implementation
must process individual chunks in order.

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>