Use esp_timer_get_time as time source for micros and mills (#1424)

esp_timer_get_time returns monotonic time in microseconds, as a 64-bit
number. It can be called from tasks and interrupts, does not use any
critical sections/mutexes, and is thread safe.

cores/esp32/main.cpp

@@ -14,7 +14,6 @@ void loopTask(void *pvParameters)
 {
     setup();
     for(;;) {
-        micros(); //update overflow
         loop();
     }
 }