policy/community-id: Populate conn$community_id in new_connection()

This wasn't possible before #3028 was fixed, but now it's safe to set
the value in new_connection() and allow other users access to the
field much earlier. We do not have to deal with connection_flipped()
because the community-id hash is symmetric.

scripts/policy/protocols/conn/community-id-logging.zeek

@@ -17,7 +17,10 @@ export {
 	};
 }
 
-event connection_state_remove(c: connection)
+module Conn;
+
+event new_connection(c: connection) &priority=5
 	{
+	Conn::set_conn(c, F);  # likely first to access :-/
 	c$conn$community_id = community_id_v1(c$id, CommunityID::seed, CommunityID::do_base64);
 	}