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.
Currently, CCoinsStats is a struct with both in-params and out-params
where the hash_type and index_requested members are the only in-params.
This change removes CCoinsStats' hash_type in-param member and adds it
to the relevant functions instead.
[META] In subsequent commits, all of CCoinsStats' members which serve as
in-params will be moved out so as to make CCoinsStats a pure
out-param struct.
