add ctx.deferCleanup (#360)

## Why

`ctx.signal.addEventListener('abort', ...)` runs _after_ the procedure
span so procedures that do things like hold a lock for the duration of
the proc drop the lock before cleanup runs

also, abort signal order is FIFO when instead cleanup should generally
be LIFO (a la Go `defer`)

ultimately this reveals a bigger issue which is that we are conflating
cancellation with cleanup

## What changed

add ctx.deferCleanup

## Versioning

- [ ] Breaking protocol change
- [ ] Breaking ts/js API change

<!-- Kind reminder to add tests and updated documentation if needed -->

Author: jackyzha0