Use random last db id, unless later overwritten by persistence.

src/database.c

@@ -144,7 +144,7 @@ int db__open(struct mosquitto__config *config)
 
 	if(!config) return MOSQ_ERR_INVAL;
 
-	db.last_db_id = 0;
+	util__random_bytes(&db.last_db_id, sizeof(db.last_db_id));
 
 	db.contexts_by_id = NULL;
 	db.contexts_by_sock = NULL;

test/unit/subs_stubs.c

@@ -225,3 +225,11 @@ void util__increment_send_quota(struct mosquitto *mosq)
 {
 	mosq->msgs_out.inflight_quota++;
 }
+
+int util__random_bytes(void *bytes, int count)
+{
+	UNUSED(bytes);
+	UNUSED(count);
+
+	return MOSQ_ERR_SUCCESS;
+}