Sync time with GPS every 30 minutes

Unless your GPS is being spoofed there isn't really a downside to
syncing more often with GPS.

I understand the RTC is very stable, but especially with powersaving now
clock drift is worse, we should sync more often.

src/helpers/sensors/MicroNMEALocationProvider.h

@@ -43,6 +43,8 @@ class MicroNMEALocationProvider : public LocationProvider {
     int _pin_en;
     long next_check = 0;
     long time_valid = 0;
+    unsigned long _last_time_sync = 0;
+    static const unsigned long TIME_SYNC_INTERVAL = 1800000; // Re-sync every 30 minutes
 
 public :
     MicroNMEALocationProvider(Stream& ser, mesh::RTCClock* clock = NULL, int pin_reset = GPS_RESET, int pin_en = GPS_EN,RefCountedDigitalPin* peripher_power=NULL) :
@@ -129,10 +131,15 @@ public :
 
         if (millis() > next_check) {
             next_check = millis() + 1000;
+            // Re-enable time sync periodically when GPS has valid fix
+            if (!_time_sync_needed && _clock != NULL && (millis() - _last_time_sync) > TIME_SYNC_INTERVAL) {
+                _time_sync_needed = true;
+            }
             if (_time_sync_needed && time_valid > 2) {
                 if (_clock != NULL) {
                     _clock->setCurrentTime(getTimestamp());
                     _time_sync_needed = false;
+                    _last_time_sync = millis();
                 }
             }
             if (isValid()) {