Faster joins: use stable identifiers

[DMRobertson]

See:

Synapse issue: #14828
Synapse PR: #14832
Complement PR: matrix-org/complement#583
GMSL PR: matrix-org/gomatrixserverlib#350
MSC: matrix-org/matrix-spec-proposals#3706
Spec diff: matrix-org/matrix-spec#1393

Also related in Synapse: #14854, #14841

[DMRobertson]

Our thinking is to just do these renames and screw backwards compat. Experimental feature etc, no promises; if you want to use faster joins both you and the resident server will have to upgrade to some minimum Synapse version with the renames.

On second thought: to assist in testing/rollout we think that it will be prudent

• to send both the old and new query parameter
• to read the new response field, falling back to the old one

But we should consider it a future maintenance task to remove the fallback, say ~6 months' time.