Move pindexFinalized in CChainState
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

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