Unsubscribe push from unsynced collections

[rfc2822]

Purpose

Adds functionality: when collections are

1. registered for Push and then
2. unselected (and thus not syncable anymore),

they shall be unregistered from the server.

Short description

PushRegistrationWorker scans for collections that are registered for Push, but not selected for synchronization, and unregisters them from Push.

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.

[sunkup]

Why do we register all collections every time? We should be able to exclude already registered collections from being registered again.

Also I take it some of the code in the worker will be extracted to dav4jvm later?

Looks good otherwise 👍

[rfc2822]

Why do we register all collections every time? We should be able to exclude already registered collections from being registered again.

The logic for re-registering before expiration is not yet implemented, so we have to re-register it every time. Also, the worker is not run regularly yet. This is also related.

Also I take it some of the code in the worker will be extracted to dav4jvm later?

I don't know whether it's necessary. We already use the POST/DELETE method from dav4jvm, and generating the message bodies is usually not done in dav4jvm. Let's have a look when it's (almost) finished.

[sunkup]

Fair, sounds good to me.