Defaults block index received time to 0 when init from a block header

Summary:
Current behavior is to set received time to the block header time. This
prevents any possibility to check if received time has been set due to
effective reception of a new block or because of a block loading from
file.

Test Plan:

make check && ./test/functionnal/test_runner.py --extended

Reviewers: #bitcoin_abc, schancel, jasonbcox, deadalnix

Reviewed By: #bitcoin_abc, schancel, jasonbcox, deadalnix

Subscribers: deadalnix, jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D2131