Include the CChainParams into the Config object.

Summary: CChainParams is passed around all over the place. This makes a very good candidate for includion into the Config object.

Test Plan:

make check

Added unittest to make sure the result are consisitents between the old and the new way.

Reviewers: freetrader, sickpig, #bitcoin_abc, kyuupichan

Reviewed By: #bitcoin_abc, kyuupichan

Differential Revision: https://reviews.bitcoinabc.org/D139