Use cstdint in seeder

Summary: As per title. no need to define a ton of custom types.

Test Plan:

make check

Reviewers: freetrader, schancel, dagurval, #bitcoin_abc, sickpig

Reviewed By: #bitcoin_abc, sickpig

Subscribers: sickpig

Differential Revision: https://reviews.bitcoinabc.org/D561