cdc: provide core changefeed data stream using notifications rather than results

[ajwerner]

Is your feature request related to a problem? Please describe.

Today the EXPERIMENTAL CHANGEFEED works by effectively creating a query in the postgres protocol with an infinite result set. This works in a relatively ergonomic way for clients but it has its downsides. One major downside is that closing the changefeed requires closing the connection. This is primarily true because the postgres protocol does not permit sending or processing messages until all of the results have been consumed. This is also a problem because cockroach does not implement the postgres cancelation protocol (#41335).

At the time when changefeeds were implemented, cockroach had no support for asynchronous notification. Using async notifications would be especially handy in order to permit multiplexing multiple changefeeds over a single connection. Such a thing would become much more handy if changefeeds supported watching portions of tables.

Jira issue: CRDB-3295

[miretskiy]

A bit of a stale issue; changefeeds now do support watching portions of tables;
@ajwerner do you still feel such a change would be useful?