Optimize write checkpoint lookups when users have none

[rkistner]

#276 included some significant optimizations to how write checkpoint lookups are performed. In general, the strategy is:

1. We watch the checkpoint_events capped collection for new checkpoints.
2. Each time we get a new checkpoint, we lookup changed buckets since the last checkpoint, changed parameter lookups, and changed write checkpoints.
3. For each user's stream, we use that as a filter to check whether or not we need to query for further data for that user.

When a new stream is opened, we need to get the initial write checkpoint for the user, before we can use the above approach to efficiently get changes. The implementation in #276 mostly handled that.

However, there was one missed case: If the user does not have any write checkpoint, it would retry the lookup on every new checkpoint. In a case of thousands of concurrent connections and no write checkpoints, we can end up with doing tens of thousands of write checkpoint lookups per second. Even if the write checkpoint collection is empty, this still ends up adding multiple megabytes/s traffic between the instance and the storage database just for sending the query and receiving the empty results.

This fixes the issue by adding a boolean to keep track of whether or not we have done the initial query, instead of using a null check on the write checkpoint.

This change has no effect on users that do have a write checkpoint.

[changeset-bot]

🦋 Changeset detected

Latest commit: 3269f76

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 11 packages

Name	Type
@powersync/service-module-mongodb-storage	Patch
@powersync/service-core	Patch
@powersync/service-image	Patch
@powersync/service-schema	Patch
@powersync/service-module-mongodb	Patch
@powersync/service-module-mysql	Patch
@powersync/service-module-postgres	Patch
@powersync/service-core-tests	Patch
@powersync/service-module-core	Patch
@powersync/service-module-postgres-storage	Patch
test-client	Patch

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[simolus3]

I'm happy with these changes 👍