s3:smbd: let session logoff close files and tcons before deleting the session

s3:smbd: let session logoff close files and tcons before deleting the session

This avoids a race in durable handle reconnects if the reconnect comes
in while the old session is still in the tear-down phase.

The new session is supposed to rendezvous with and wait for destruction
of the old session, which is internally implemented with
dbwrap_watch_send() on the old session record.

If the old session deletes the session record before calling
file_close_user() which marks all file handles as disconnected, the
durable handle reconnect in the new session will fail as the records are
not yet marked as disconnected which is a prerequisite.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=13549

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