compareTo method
- GenericTransaction other
override
    Compares this object to another object.
Returns a value like a Comparator when comparing this to other.
That is, it returns a negative integer if this is ordered before other,
a positive integer if this is ordered after other,
and zero if this and other are ordered together.
The other argument must be a value that is comparable to this object.
Implementation
@override
int compareTo(GenericTransaction other) {
  if (isPending) {
    if (other.isPending) return 0;
    return -1;
  }
  if (other.isPending) return 1;
  return -timeMilli.compareTo(other.timeMilli);
}