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.

The indexing codepath logic in node/coinstats.cpp is simple enough to be
moved into CoinStatsIndex::LookUpStats, avoiding an additional layer of
function calls. Callers are modified accordingly.

Also, add 2 missed BOOST_CHECKs to the coinstatsindex_initial_sync unit
test.

As mentioned in a previous commit, the hashing codepath can now be moved
to a separate file. This decouples callers that only rely on the hashing
codepath from the indexing one.

This is key for libbitcoinkernel, which needs to have the CoinsStats
hashing codepath for AssumeUTXO, but does not wish to be coupled with
indexes.

Note that only the .cpp file is split in this commit, the header files
will be split in a subsequent commit and the #includes to
node/coinstats.h will be adjusted to only #include the necessary
headers.

Most of this commit is pure-move.

After this change:

- kernel/coinstats.h
    -> Contains declarations for:
       - enum class CoinStatsHashType
       - struct CCoinsStats
       - GetBogoSize(...)
       - TxOutSer(...)
       - ComputeUTXOStats(...)
- node/coinstats.h
    -> Just GetUTXOStats, which will be removed as we change callers to
       directly use the hashing/indexing codepaths in future commits.

Removes a circular dependency, horray!

Introduces a new kernel:: namespace and move all of src/kernel/coinstats
under it.

In the verify script, lines like:

line="$(grep -n 'namespace node {' -- src/kernel/coinstats.h | tail -n1 | cut -d: -f1)"
sed -i -e "${line}s@namespace node {@namespace kernel {@" -- src/kernel/coinstats.h

Are intended to replace only the last instance of "namespace node" with
"namespace kernel", this is to avoid replacing forward declarations of
things inside the node:: namespace.

-BEGIN VERIFY SCRIPT-
sed -E -i 's@namespace node@namespace kernel@g' -- src/kernel/coinstats.cpp

line="$(grep -n 'namespace node {' -- src/kernel/coinstats.h | tail -n1 | cut -d: -f1)"
sed -i -e "${line}s@namespace node {@namespace kernel {@" -- src/kernel/coinstats.h

line="$(grep -n '// namespace node' -- src/kernel/coinstats.h | tail -n1 | cut -d: -f1)"
sed -i -e "${line}s@// namespace node@// namespace kernel@" -- src/kernel/coinstats.h

things='(CCoinsStats|CoinStatsHashType|GetBogoSize|TxOutSer|ComputeUTXOStats)'
git grep -lE 'node::'"$things" | xargs sed -E -i 's@node::'"$things"'@kernel::\1@g'
sed -E -i 's@'"$things"'@kernel::\1@g' -- src/node/coinstats.cpp src/node/coinstats.h
sed -E -i 's@BlockManager@node::\0@g' -- src/kernel/coinstats.cpp
-END VERIFY SCRIPT-

This is the "fruit of our labor" for this patchset.
ChainstateManager::PopulateAndValidateSnapshot can now directly call
ComputeUTXOStats(...).

Our consensus engine is now fully decoupled from all indices.

See the src/Makefile.am for some satisfying removals.