Update seeder to use int64_t instead of time_t for Darwin

Summary:
On MacOS X, time_t is a typedef for long. In order to
compile bitcoin-seeder on MacOS X we need to not
use time_t, and instead use int64_t.

Test Plan: make VERBOSE=1 check

Reviewers: kyuupichan, deadalnix, freetrader, #bitcoin_abc

Reviewed By: kyuupichan, deadalnix, #bitcoin_abc

Subscribers: zquestz

Differential Revision: https://reviews.bitcoinabc.org/D579