This is a commit in preparation for the next few commits. The functions
are moved to methods to avoid their re-declaration for the purpose of
passing in BlockManager options.
The functions that were now moved into the BlockManager should no longer
use the params as an argument, but instead use the member variable.
In the moved ReadBlockFromDisk and UndoReadFromDisk, change
the function signature to accept a reference to a CBlockIndex instead of
a raw pointer. The pointer is expected to be non-null, so reflect that
in the type.
To allow for the move of functions to BlockManager methods all call
sites require an instantiated BlockManager, or a callback to one.
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