The "virtual dirty" term is not very informative. "Virtual" means
"not real", but it doesn't say in which way it isn't real.
In this case, virtual dirty refers to real dirty memory, minus
the portion of memtables that has been written to disk (but not
yet sealed - in that case it would not be dirty in the first
place).
I chose to call "the portion of memtables that has been written
to disk" as "spooled memory". At least the unique term will cause
people to look it up and may be easier to remember. From that
we have "unspooled memory".
I plan to further change the accounting to account for spooled memory
rather than unspooled, as that is a more natural term, but that is left
for later.
The documentation, config item, and metrics are adjusted. The config
item is practically unused so it isn't worth keeping compatibility here.
