[Push] Support sync-token

[rfc2822]

Currently DAVx⁵ enqueues a sync worker whenever it receives a push message.

However the sync-token property of the push message should be checked, and if the sync-token is already up-to-date, no sync should be enqueued.

This avoids redundant sync jobs which are caused by own uploads:

1. Upload by DAVx⁵ changes sync-token to new-sync-token. Server sends push message, but it will take a few seconds to arrive.
2. DAVx5 runs sync, stores current sync-token=new-sync-token.
3. DAVx⁵ receives push message with sync-token=new-sync-token. Because the collection is already up-to-date, no new sync worker is enqueued.

Possible way to implement:

• Provide some way to get a LocalCollection from the collection ID, probably as method of LocalDataStore (and then select the data store according to the collection type).
• If we have a LocalCollection that has been updated, we can compare the sync-token with localCollection.lastSyncState and ignore the push if the collection is already up-to-date.

Depends on bitfireAT/ical4android#171
Depends on https://github.com/bitfireAT/davx5/issues/603 (not strictly, but should be done afterwards because the database collection ID of local collections is required)

[github-actions]

This PR/issue depends on:

• Add _sync_id for collections ical4android#171