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