Don't register ability to hover, request colors, etc… more than once

[thecrypticace]

We're currently re-registering "common" capabilities per workspace folder. This causes VSCode to make multiple requests to the server for things like hover causing higher CPU usage. This then results multiple hovers being displayed. What's worse is that for some scenarios where the server has to internally do a restart the old registrations weren't getting disposed of because of a race condition when calling it concurrently for multiple folders.

I've done two things to address this:

• Common capability registration will only happen after all project folders have been initialized — hopefully this doesn't cause any problems (though if it does it's revealing a bigger underlying one)
• Common capabilities will be explicitly disposed before registering again

This should mean that this only happens one time per server initialization.

Fixes #1371