changefeedccl: Add changefeed_creation_timestamp function

[miretskiy]

Add changefeed_create_timestamp function, which returns changefeed creation timestamp.

Changefeed transformations restrict access to some of the standard functions, including now().
Without such function, it is difficult to express
changefeeds that want to emit events restricted
by time. This function makes it possible to do
this. For example, to create a changefeed that
emits events from the accounts table that
last_withdrawal happen starting 12 hours ago,
one could do:

CREATE CHANGEFEED ... AS
SELECT * FROM accounts WHERE last_withdrawal >
changefeed_create_timestamp() - interval '12 hours'

Epic: CRDB-17161

Release note (enterprise change): Changefeed expressions support changefeed_create_timestamp function.

[cockroach-teamcity]

This change is 

[shermanCRL]

Bikeshed, I think create sounds like a verb/command. Maybe created or creation?

[miretskiy]

Bikeshed, I think create sounds like a verb/command. Maybe created or creation?

easy enough to change -- done.

[jayshrivastava]

Maybe cdc_now_timestamp() or cdc_now() aligns better with the other cdc internal thing we have (cdc_prev). Also, using now somewhere in the name aligns more with the standard now() fn.

Reviewed 5 of 6 files at r1, all commit messages.
Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @miretskiy)

---

pkg/ccl/changefeedccl/cdceval/expr_eval.go line 115 at r1 (raw file):

		rowCh: make(chan tree.Datums, 1),
	}
	e.rowEvalCtx.startTime = statementTS

It would be a neater to make a constructor for rowEvalCtx which does this.

---

pkg/ccl/changefeedccl/cdceval/expr_eval.go line 452 at r1 (raw file):

func (e *familyEvaluator) setupContextForRow(ctx context.Context, updated cdcevent.Row) error {
	e.rowEvalCtx.ctx = ctx
	e.rowEvalCtx.updatedRow = updated

Consider putting these lines in a function func (rec *rowEvalContext) updateForRow(ctx context.Context, updated cdcevent.Row).

Code quote:

	e.rowEvalCtx.ctx = ctx
	e.rowEvalCtx.updatedRow = updated

---

pkg/ccl/changefeedccl/cdceval/functions.go line 133 at r1 (raw file):

	"changefeed_creation_timestamp": cdcTimestampBuiltin(
		"changefeed_creation_timestamp",
		"Returns changefeed creation time",

nit: transaction timestamp or timestamp would be more clear.

Code quote:

eturns changefeed creation time

[miretskiy]

bors r+

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)