Port to GDateTime
Drop our date/time helpers in favor of GDateTime. This has several advantages:
- Less code to maintain
- It behaves correctly on arches/platforms where time_t is still 32bit
- Fixes our limitations at parsing sub-millisecond precision, even though we handle up to microseconds
There is one disadvantage though:
- GDateTime and SQLite slightly disagree about the supported range. The lower limit for our date handling moves from 0000-01-01 to 0001-01-01. Upper limit stays at 9999-12-31.
The disadvantage is too small and unlikely to have real consequences, so it's considered a minor change.