Summary

Note: this is a new PR that addresses a compile-time deprecation warning that caused #1626 to be reverted.
This change adds webhook support to enable real-time syncing of remote updates. It takes advantage of the webhooks functionality provided by Microsoft. This fixes #1620.

How does it work

1. The onedrive process stands up a local web server to receive incoming notifications (to be sent by Microsoft)
2. The onedrive process registers the public-facing url of the web server with Microsoft, via the POST /subscriptions endpoint.
3. When there is an remote update, Microsoft sends a notification request to the pre-registered url.
4. The onedrive process receives the notification request, and kicks off a new delta sync loop to fetch updates.

Code changes

1. onedrive.d contains the bulk of logic to manage the local web server and to register/renew subscriptions with Microsoft. It is also responsible for processing incoming notification requests. When such requests are received, it sends signals to the main sync loop.
2. main.d contains the logic to process incoming signals and execute delta sync to fetch updates.
3. The embeded web server is made possible via cgi.d from adamdruppe/arsd
4. New config options are added to config.d and also documented.

Testing

Unfortunately I could only test this manually with a very straightforward setup: personal OneDrive account with no shared folders. More testing is welcome!

What is next

Currently the public-facing url has to be statically configured. This should work well for most users either with static public IP or with dynamic DNS services. We could consider adding UPnP support to allow dynamic configuration via automatic port forwarding.