Pass config down InitBlockIndex and remove CChainParams argument from ActivateBestChain
Summary: As per title. This is a step toward using the Config object rather than passing ad hoc stuff around.
Test Plan:
make check
Reviewers: freetrader, sickpig, kyuupichan, #bitcoin_abc
Reviewed By: freetrader, #bitcoin_abc
Differential Revision: https://reviews.bitcoinabc.org/D162