s3/smbd: enable processing SMB2 requests async internally

s3/smbd: enable processing SMB2 requests async internally

The idea is to allow the implementation of an SMB2 request to tell the
main SMB2 processing engine that it wants to handle a requests
asynchronously internally.

This has two use cases:

- it allows (internal) async processing of compound requests that would
  otherwise be rejected by the SMB2 processing engine

- it preserves sync semantics at the SMB layer, some clients might not
  expect arbitrary SMB2 requests going async

Not used for now, will be used in laters commit for async SMB2 FIND
requests.

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