Currently, the p2p and rpcserver handlers that deal with compact filters
essentially duplicate the logic for dealing with the associated header
proof and are also required to understand the structure of the proof in
the form of knowing the associated index. This is not ideal since there
is no guarantee that a given index will always be the same through
consensus changes.
With those points in mind, this reworks the header proof logic slightly
to consolidate it by introducing a new HeaderProof struct to the
blockchain that houses a header commitment inclusion proof and
associated proof index and updating the FilterByBlockHash method to
return the header proof along with the filter and updates the p2p and
rpcserver handlers and associated interfaces and tests accordingly.
The result is that the aforementioned p2p and rpcserver handlers for
obtaining compact filters no longer need to understand anything about
the proofs. It is also useful for any future header commitments too
since the same logic will hold true for those instances as well.
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