[wip] prototyping for rangefeed scalability work

[wenyihu6]

Just prototyping - This patch reduces registration goroutines and removes r.buf at rangefeed level.

TODO:

• rationalize memory accounting
• tests
• is it okay to send same registration error twice

Shutdown logic: https://docs.google.com/document/d/1rQzVYnI4XuWloenEYa_9cvQ5aVYPlhR2cb8IAQKo6_M/edit?usp=sharing