Request thread head via Cafe

[andrewxhill]

Allow textile-lite clients (or any client really) to request the latest head of a thread from a Cafe.

It might mean that when a thread is created the author requests head tracking or maybe it is out of the box for any thread where the user has a cafe. @sanderpick will know the details better.

• Worth considering the inverse: Post to thread via Cafe. This could be pure proxy post (let the cafe sign the add but reference the requestor or signed request) or a pass through, where the requestor sends a fully signed block.

[leshokunin]

This would be great! It came up yesterday as a blocker for us.
When we write a new article on Epona, we create a new entry in a thread. Currently, we don't have a way to have a permalink for that thread that would show the latest entry we saved. We can only show a specific version. This would allow us to make permalinks and make sharing easier!