MT#6969 PoC message to resync SPA.

- needs adations in impresence template:
 - double-auth not possible, so no auth on proxy to be done
 - handle Event: check-sync differently (RELAY_IMP), otherwise
   it goes into RLS handling by default.

etc/check-sync.sip

@@ -0,0 +1,8 @@
+NOTIFY sip:development301@pbx1.demo.sipwise.com SIP/2.0
+To: <sip:development301@pbx1.demo.sipwise.com>
+From: <sip:development301@pbx1.demo.sipwise.com>;tag=12345
+Call-ID: resynctest12345
+CSeq: 1 NOTIFY
+Subscription-State: active
+Event: check-sync
+Content-Length: 0