CVE-2021-3738 s4:rpc_server/samr: make use of dcesrv_samdb_connect_as_*() helper

CVE-2021-3738 s4:rpc_server/samr: make use of dcesrv_samdb_connect_as_*() helper

This avoids a crash that's triggered by windows clients using
handles from samr_Connect*() on across multiple connections within
an association group.

In other cases is not strictly required, but it makes it easier to audit that
source4/rpc_server no longer calls samdb_connect() directly and also
improves the auditing for the dcesrv_samdb_connect_as_system() case.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14468

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>