This MR changes the API URL prefix from
/_matrix/client/v3/, meaning we use version 1.3 of the Matrix specification.
An advantage of this, is that we stop using
/initialSync to get a pagination pointer, and instead just call
/initialSync is slow (like, many-seconds slow), so that's really nice!
I also heard that the Conduit homeserver doesn't implement the (deprecated)
/initialSync endpoint - so we need this for Cactus to work with Conduit.
But this probably breaks one some old homeservers. I don't know which versions and how many, though... We should find out before merging. I guess the cool thing to do would be to detect which APIs the homeserver implements, and act accordingly. Is it worth doing?