[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

[blathers-crl[bot]]

Your pull request contains more than 1000 changes. It is strongly encouraged to split big PRs into smaller chunks.

_{:owl: Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[cockroach-teamcity]

This change is 

[blathers-crl[bot]]

Your pull request contains more than 1000 changes. It is strongly encouraged to split big PRs into smaller chunks.

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

_{:owl: Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[wenyihu6]

Was just for prototyping.