Add Read Cache and Stats

- baro.cache_ttl_ms avoids multiple reads
- by default, cache is off (cache_ttl_ms=0)
- stats are updated (similar to how prometheus-sensors does this)

src/main.c

@@ -217,6 +217,8 @@ int main() {
   */
   if (!(baro=mgos_barometer_create_i2c(i2c, 0x60, BARO_MPL115))) {
     LOG(LL_ERROR, ("Cannot create MPL115 barometer"));
+  } else {
+    mgos_barometer_set_cache_ttl(baro, 5000);
   }
 
   for (;;) {