Concurrent storage write batch support

[rkistner]

This modifies the APIs and implementation of BucketStorageBatch to allow using multiple batches concurrently without introducing consistency issues. The first goal of this is to allow reading the replication stream while snapshotting - see #426 for a Postgres implementation.

The actual writes to the database still rely on exclusive database locks to a large extent, so there isn't true concurrency yet. But it does allow running a replication stream concurrently with replication snapshots without breaking.

Specifically, this changes:

1. Rely less on local variables in the batch, using the database state instead.
2. Remove the distinction between commit() and keepalive(). The only remaining difference is that keepalive is essentially commit({allowEmptyCommit: true}).
3. snapshot_done is now explicitly set after completing a snapshot, rather than setting it automatically on commit. No commit will go through until snapshot_done is set.
4. current_data deletes are now soft-deletes (storing an empty document/row instead of deleting the document/row), converted into hard deletes on the next commit. We need this to consistently handle snapshots while streaming.
5. Fix concurrency issues for flushing and committing batches, especially in Postgres storage (previously not an issue since we wouldn't ever do this concurrently).
6. Change SourceTable.id to be strongly typed, specifically checking for string in Postgres storage and ObjectId in MongoDB storage. This unfortunately had a big knock-on effect in tests, including now having different checksums in postgres storage vs mongodb storage.

Other more minor changes:

1. Reduce migration logs in tests.
2. Rely less on a split between initial snapshot and streaming replication in Postgres tests.
3. Add a cause to ReplicationAbortedError.

[changeset-bot]

🦋 Changeset detected

Latest commit: 5c18559

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

This PR includes changesets to release 17 packages

Name	Type
@powersync/service-module-postgres-storage	Minor
@powersync/service-module-mongodb-storage	Minor
@powersync/service-core-tests	Minor
@powersync/service-module-postgres	Minor
@powersync/service-errors	Minor
@powersync/service-module-mongodb	Minor
@powersync/service-core	Minor
@powersync/service-module-mysql	Minor
@powersync/lib-services-framework	Minor
@powersync/service-schema	Minor
@powersync/service-module-mssql	Patch
@powersync/service-image	Minor
@powersync/service-module-core	Patch
test-client	Patch
@powersync/service-rsocket-router	Patch
@powersync/lib-service-mongodb	Patch
@powersync/lib-service-postgres	Patch

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