[seeder] Use GetTime() utility function instead of accessing ctime directly

Summary:
This does two things:

1. Introduces mocktime for future seeder functional testing
2. Enforces a bit width for time, since time_t is not the same on all platforms.

Test Plan:

ninja check

Run cross builds on CI

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8642