Merge #16194: refactor: share blockmetadata with BlockManager

682a1d0f2004d808b87b3106d0dfae547005e638 refactoring: remove mapBlockIndex global (James O'Beirne)
55d525ab9004631d30dcc60a1ec5d9cd6c6afe56 refactoring: make pindexBestInvalid internal to validation.cpp (James O'Beirne)
4ed55dfcd7894fd5ba6395f244a17ab1f8e786d4 refactoring: add block_index_candidates arg to LoadBlockIndex (James O'Beirne)
613c46fe9e39f55b0f0daa18fee20b4120db2539 refactoring: move block metadata structures into BlockManager (James O'Beirne)

Pull request description:

  This is part of the [assumeutxo project](https://github.com/bitcoin/bitcoin/projects/11):

  Parent PR: #15606
  Issue: #15605
  Specification: https://github.com/jamesob/assumeutxo-docs/tree/2019-04-proposal/proposal

  ---

  Under an assumeutxo model, we have multiple CChainState instances in use at once in order to support background validation. Currently, each CChainState instance has its own mapBlockIndex, a collection of linked block headers, in addition to a few other data structures that are related to maintenance of the block tree but not necessarily to any given chainstate.

  In order to avoid duplicating this data across chainstates, this change moves chainstate-agnostic block metadata (and related behavior) into a class, `BlockManager`. Chainstates are parameterized with a reference to a blockmanager instance and in practice they share the same instance.

  Most of this change is conceptually move-only, though the diff is somewhat muddled. The first commit can be reviewed slightly more easily with `--color-moved=dimmed_zebra`. Admittedly, that commit is pretty unwieldy; I tried to split it up after the fact with `git add --patch`, but that was difficult because of git's inability to split hunks past a certain point. Some of the moves also ended up being obscured when done over separate commits.

ACKs for top commit:
  MarcoFalke:
    ACK 682a1d0f2004d808b87b3106d0dfae547005e638
  ryanofsky:
    utACK 682a1d0f2004d808b87b3106d0dfae547005e638, only changes since last review were rebase and fixing conflict on a moved line
  ariard:
    utACK 682a1d0. Most of the changes are move-only, with main problem being to avoid creating circular dependencies between `BlockManager` and `CChainState`. Tested, comments are mostly nits, feel free to ignore them

Tree-SHA512: 738d8d06539ba53acf4bd2d48ae000473e645bbc4e63d798d55d247a4d5a4f781b73538ed590f6407be9ab402ea9d395570ea20bff0a4b9ce747bcc1600c5108

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