This change has multiple small components.
First, the chunk size is bumped to 1GB in order to avoid creating excessive
VMAs in the Sentry, which can lead to VMA exhaustion (and hitting limits).
Second, gap-tracking is added to the usage set in order to efficiently scan
for available regions.
Third, reclaim is moved to a simple segment set. This is done to allow the
order of reclaim to align with the Allocate order (which becomes much more
complex when trying to track a "max page" as opposed to "min page", so we
just track explicit segments instead, which should make reclaim scanning
faster anyways).
Finally, the findAvailable function attempts to scan from the top-down, in
order to maximize opportunities for VMA merging in applications (hopefully
preventing the same VMA exhaustion that can affect the Sentry).
PiperOrigin-RevId: 315009249
9aaca5a6da