Rebuild spool as a swap space

[Dav1dde]

We should start treating the spool as a swap, at a certain point of managed envelopes we should start buffering to disk and treat the disk as a swap space.

We've seen in the past that the current spooling behaviour is not enough, especially in lower memory setups. It also causes confusion, people expect the spool to work like a swap but then it doesn't.

Related to: https://github.com/getsentry/team-ingest/issues/131