Move pindexFinalized in CChainState

Summary: As per title. This is a chain state and there are no reason for it not to be in there.

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4582