refactor: Use type-safe time in txorphanage

Summary:

This allows to remove manual conversions like multiplication by 60, and uses a type-safe type instead of a raw int64_t.

Backport of core#30170.

Depends on D16498.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16501