in lieu of ::BlockIndex().
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Depends on D6968
Concludes backport of Core PR16194
Notes: This backport originally changes LookupBlockIndex from being defined inline in validation.h to declared there and defined in validation.cpp.
Since we have that function defined in chain.h, I have chosen to defer to the change in the backport to stick closer to Core.
That has the unfortunate side effect of bringing back the checkpoints -> validation -> checkpoints circular dependency, that I plan on resolving by backporting Core PR15655, if this diff is landed as is.