[backport#16443 1/3] refactor: pcoinsTip -> CChainState::CoinsTip()
Summary:
This aliasing makes subsequent commits easier to review; eventually CoinsTip()
will return the CCoinsViewCache managed by CChainState.
https://github.com/bitcoin/bitcoin/pull/16443/commits/fae6ab6aed3b9fdc9201bb19a307dfc3d9b89891
Partial backport of Core PR16443
Test Plan:
ninja check check-functional
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D7580