Replace hardcoded future block time with preexisting constant

Summary:
Hard coded magic numbers are bad. Preexisting constant:
./chain.h:23:static const int64_t MAX_FUTURE_BLOCK_TIME = 2 * 60 * 60;

Test Plan: make check

Reviewers: deadalnix, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1081