Due to boltdb behavior, dbSizeInUse can be unequal to dbSize even when
dbSize is growing in some conditions. The possible gap between the sizes
is up to a couple of pages.
Closes#9302
Existing dbSize shows physically allocated DB size and the backend
(boltdb) won't shrink it after a compaction until a user runs the defrag command.
The new dbSizeInUse shows the DB size that excludes free pages created
by compactions so that users can see the actual DB usage. dbSize >=
dbSizeInUse is always true.
Note that dbSizeInUse shows a page-based size and not byte level usage.
Gyuho Lee