Support OPFSWriteAheadVFS, concurrent reads

[simolus3]

This adds an option to use the new OPFSWriteAheadVFS VFS under WASQLiteVFS.

Unlike the existing VFS implementations, this one supports concurrent reads through a write-ahead log. To make use of that in the PowerSync SDK, we use a simple concurrency setup:

1. When the VFS is enabled, we spawn two dedicated workers instead of one. On the second worker, we open the database in read-only mode.
2. In AsyncConnectionPool, we may use the read worker for read locks if available.

This is probably not an optimal setup, but since it's only used as a new feature we can tune this after an initial release. Some obvious concerns:

• Should we support more than one read connection? We could wrap multiple connections into a pool using our Semaphore utility, but I'm a bit worried about resource usage. Especially if users open many tabs at once, we probably don't want each tab to keep multiple read workers open (they listen to a broadcast channel to catch up with the WAL even when idle). We could investigate support for a read pool resizing based on how many queries a tab runs.
• Selecting a connection: Our other connection pools delegate all reads to dedicated read connections. Since we only have a single read connection here, it makes sense to keep using the write connection for reads too (so that we can speed up read-heavy workloads). In this PR, I'm using whatever connection has fewer in-flight requests. That's a pretty terrible metric, a long-running write transaction would block some reads scheduled for the write connection in this case. Better concurrency control would definitely help here.

[changeset-bot]

🦋 Changeset detected

Latest commit: b56020d

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

This PR includes changesets to release 2 packages

Name	Type
@powersync/web	Minor
@powersync/diagnostics-app	Patch

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