Setting a large -dbcache size postpones the index writes until the coins cache size exceeds the specified limit.
This causes the final flush after manual termination to seemingly hang forever (e.g. tens of minutes for 20 GiB);
Now that the dbcache upper cap has been lifted (D18577), this will become even more apparent, so a warning will be shown when large UTXO sets are flushed (currently >1 GiB).
Note that the related BCLog::BENCH units were also converted to KiB from kB to unify the bases.
Co-authored-by: Cory Fields <cory-nospam-@coryfields.com>
This is a backport of core#31534