py:dcerpc/raw_protocol: add tests to demonstrate how security context multiplexing...

py:dcerpc/raw_protocol: add tests to demonstrate how security context multiplexing works

Important things are this:
- It's not required to use the bind time feature negotiation in order
  to use it, it's only a hint for the client, but nothing is really
  negotiated, unlike the request multiplexing with the
  DCERPC_PFC_FLAG_CONC_MPX.
- There's special handling related to AUTH_LEVEL_CONNECT
  and requests without auth trailer
- An security context is identified by the unique
  tuple of auth_type, auth_level and auth_context_id (all together!),
  not just the auth_context_id.
- There's a limit of 2049 explicit authentication contexts.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=7113
BUG: https://bugzilla.samba.org/show_bug.cgi?id=11892

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>