s3/vfs: add SMB_VFS_OFFLOAD_READ_SEND/RECV

s3/vfs: add SMB_VFS_OFFLOAD_READ_SEND/RECV

Add SMB_VFS_OFFLOAD_READ_SEND an SMB_VFS_OFFLOAD_READ_RECV.

This paves the way for supporting server-side copy-chunk with source and
destination file-handles on different shares. It can be used to
implement copy offload fsctl in the future, but for now this will be
used as a mere copy-chunk replacement.

SMB_VFS_OFFLOAD_READ generates a token that associates an fsp with the
token and stores the fsp in a in-memory db.

Initially only a copy-chunk resume key fsctl is supported. In the future
this can be enhanced to support real offload fsctl.

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>