Refs #6148
Separates disk usage into two cases: Allocated and used.
Since we use both reserve and recycled segments, both
which are not actually filled with anything at the point
of waiting.
Also refuses to recycle segments or increase reserve size
if our current disk footprint exceeds threshold.
And finally uses some initial heuristics to determine when
we should suggest flushing, based on disk limit, segment
size, and current usage. Right now, when we only have
a half segment left before hitting used == max.
Some initial tests show an improved adherence to limit
though it will still be exceeded, because we do _not_
force waiting for segments to become cleared or similar
if we need to add data, thus slow flushing can still make
usage create extra segments. We will however attempt to
shrink disk usage when load is lighter.
Somewhat unclear how much this impacts performance
with tight limits, and how much this matters.
v2:
* Add some comments/explanations
v3:
* Made disk footprint subtract happen post delete (non-optimistic)
5d044ab74e