The ever contentious capabilities for new sessions

Hi,

We spend an awful lot of time at F2F sessions on capabilities, but I think
Jason Leyba's current suggestion nails almost all the points that have been
raised in meetings and in person:

https://github.com/w3c/webdriver/pull/327

Notably:

   - Address the desire for simple processing of capabilities by end nodes,
   with exact matches only.
   - Makes it possible to describe several different possibilities.
   - Has a different set of blob keys, meaning that the protocol handshake
   between OSS, the original spec text, and the new spec text can be done
   unambiguously (esp. if end nodes hold to Postel's Law)
   - Makes an effort to reduce data being sent across the wire through the
   use of the "required capabilities" being merged with "first match".

The biggest downside from my point of view is that this is hard to make
100% backward compatible with the widespread use of selenium, but we could
handle iterating over the values on the local end until we ship Selenium 4.

If there are no serious objections, I think that this is a sufficiently
improved basis for discussion to be worth merging into the spec ready for
the f2f.

Simon