[counter] Remove stored counter value on zero value

Rationale: The counter store is of `map[string]int64` and therefor for
an non-existent counter always `0`. As that is the default behaviour we
don't need to waste storage space by storing counters being `0`.

Signed-off-by: Knut Ahlers <knut@ahlers.me>

store.go

@@ -294,7 +294,11 @@ func (s *storageFile) UpdateCounter(counter string, value int64, absolute bool)
 		value = s.Counters[counter] + value
 	}
 
+	if value == 0 {
+		delete(s.Counters, counter)
+	} else {
 		s.Counters[counter] = value
+	}
 
 	return errors.Wrap(s.Save(), "saving store")
 }