SSU2: MTU and other fixes

Use minimum MTU for IPv6 session confirmed to avoid PMTU issues
Set default MTU for known IPv6 tunnel brokers
Check for s mismatch in session confirmed RI
Don't put DateTime block after Termination block
Fix first message failed check
Log sent data packet size
Log tweaks

history.txt

@@ -1,3 +1,18 @@
+2022-07-17 zzz
+ * SSU2:
+   - More MTU fixes
+   - Verify static key in RI
+   - Don't put DateTime block after Termination block
+   - Fix first message fail check
+
+2022-07-15 zzz
+ * SSU2:
+   - MTU fixes
+   - Send retry with termination on clock skew
+
+2022-07-12 zzz
+ * SSU2: Fail session if first outbound message fails
+
 2022-07-10 zzz
  * SSU2: Fix NACK handling