validation: improve performance of CheckBlockIndex

by not saving all indexes in a std::multimap, but only
those that are not part of the best header chain.
The indexes of the best header chain are stored in a vector,
which, in the typical case of a mostly linear chain with
a few forks, results in a much smaller multimap, and increases
performance noticeably for long chains.

This does not change the actual consistency checks that are being
performed for each index, just the way the block index tree is
stored and traversed.

Co-authored-by: Ryan Ofsky <ryan@ofsky.org>

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