coinstats: Separate hasher/index lookup codepaths

Split out ComputeUTXOStats and LookupUTXOStatsWithIndex from
GetUTXOStats, since the hashing and indexing codepaths are quite
disparate in practice.

Also allow add a constructor to CCoinsStats for it to be constructed
from a a block height and hash. This is used in both codepaths.

Also add a note in GetUTXOStats documenting a behaviour quirk that
predates this patchset.

[META] This allows the hashing codepath to be moved to a separate file
       in a future commit, decoupling callers that only rely on the
       hashing codepath from the indexing one. This is key for
       libbitcoinkernel, which needs to have the hashing codepath for
       AssumeUTXO, but does not wish to be coupled with indexes.

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