Export boot timestamp, update docs and examples to use it

src/timer.h

@@ -21,4 +21,3 @@ void mg_timer_init(struct mg_timer **head, struct mg_timer *timer,
                    void *arg);
 void mg_timer_free(struct mg_timer **head, struct mg_timer *);
 void mg_timer_poll(struct mg_timer **head, uint64_t new_ms);
-bool mg_timer_expired(uint64_t *expiration, uint64_t period, uint64_t now);