Purpose

This PR checks the sync-token of incoming push messages. If the local collection is already up-to-date, no sync worker is enqueued. This is usually the case if local changes are uploaded:

1. DAVx⁵ uploads the changes during a sync → sync token changes to ST1, push message is sent
2. DAVx⁵ continues the sync → collection is now synced and in state ST1
3. DAVx⁵ receives the push message with sync-token=ST1.

Now it doesn't enqueue a new sync, because it's already up-to-date.

Short description

When receiving a push message, the sync token is checked for the given collection before a sync is enqueued.

Checklist

• The PR has a proper title, description and label.
• I have self-reviewed the PR.
• I have added documentation to complex functions and functions that can be used by other modules.
• I have added reasonable tests or consciously decided to not add tests.

Depends on #1272