Fix rare checkpoint hang

Summary

Fixes a theoretical, rare issue whereby multiple checkpoints can be reported before the next one is awaited, resulting in an entire checkpoint being missed.

At best, this can cause the core loop to hang, and at worst make strange decisions.

[!NOTE] The current checkpoints that are supported mean that we never see this issue. However, this PR guards us against altering the checkpoints in the future and not accounting for this quirk.

For example:

await checkpointPromise resolves
yield reports checkpoint
Checkpoint B is found; setCheckpoint is called, which reassigns checkpointPromise
Checkpoint C is found; setCheckpoint is called, which reassigns checkpointPromise
Core loop while ticks over and hits await checkpointPromise, which resolves with Checkpoint C
We've missed Checkpoint B

Checklist

[ ] ~~Added a docs PR that references this PR~~ N/A Internal fix
[ ] ~Added unit/integration tests~ N/A Existing cover this fine; the unit tests for this would be much too abstract
[x] Added changesets if applicable

inngest / inngest-js

Fix rare checkpoint hang #622

Summary

Checklist

🦋 Changeset detected