add config option to make absolute_time_t always a uint64_t

[kilograham]

The intention was that people would always use the macros (e.g. to_us_since_boot, so the difference in implementation would not matter.

As stated in the docs the reason for making absolute_time_t a separate type was to avoid confusion between timeouts and longs.

Frankly, if people were using the macros, it is probably fine to make the default be that everything is a uint64_t.