Meta-issue tracking the work required to get core changefeeds production-ready. Currently this includes at least:

• figure out the final syntax. CREATE CHANGEFEED is awkward given that it doesn't create anything
• figure out the return types. key/value is awkward to work with, but it's not clear how we'd return the actual columns if the feed is over multiple tables. maybe require one feed per table?
• support pgwire cancellation. calling rows.Close should shut down the changefeed
• fix pgwire flushing. currently worked around with the COCKROACH_CONN_RESULTS_BUFFER_BYTES env var
• make sure this plays well with the optimizer. should work with[] in queries SELECT * FROM [CREATE CHANGEFEED ...] WHERE ...