Fixed logging block diff times so that the order of blocks being received doesn't change what is logged

Summary: Discovered this while debugging tests for D1442.

Test Plan:
With D1442 applied:
build/test/functional/test_runner.py sendheaders-selfish-mining.py and
examine the logs at /tmp/bitcoin_test_runner_xxx/sendheaders-selfish-mining_xxx/node0/regtest/debug.log.

Note that the tmp files get cleaned up if the test passes, so make it fail
with assert() at the end of sendheaders-selfish-mining.py to see these logs.

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1511