Perform throttling for concurrent build index which is done outside transaction (#5048)

See 
https://neondb.slack.com/archives/C03H1K0PGKH/p1692550646191429

## Problem

Build index concurrently is writing WAL outside transaction.
`backpressure_throttling_impl` doesn't perform throttling for read-only
transactions (not assigned XID).
It cause huge write lag which can cause large delay of accessing the
table.

## Summary of changes

Looks at `PROC_IN_SAFE_IC` in process state set during concurrent index
build.
 
## Checklist before requesting a review

- [ ] I have performed a self-review of my code.
- [ ] If it is a core feature, I have added thorough tests.
- [ ] Do we need to implement analytics? if so did you add the relevant
metrics to the dashboard?
- [ ] If this PR requires public announcement, mark it with
/release-notes label and add several sentences in this section.

## Checklist before merging

- [ ] Do not forget to reformat commit message to not include the above
checklist

---------

Co-authored-by: Konstantin Knizhnik <knizhnik@neon.tech>
Co-authored-by: Heikki Linnakangas <heikki@neon.tech>

pgxn/neon/walproposer.c

@@ -2539,8 +2539,15 @@ backpressure_throttling_impl(void)
 	? PrevProcessInterruptsCallback()
 	: false;
 
-	/* Don't throttle read only transactions and wal sender. */
-	if (am_walsender || !TransactionIdIsValid(GetCurrentTransactionIdIfAny()))
+	/*
+	 * Don't throttle read only transactions or wal sender.
+	 * Do throttle CREATE INDEX CONCURRENTLY, however. It performs some
+	 * stages outside a transaction, even though it writes a lot of WAL. 
+	 * Check PROC_IN_SAFE_IC flag to cover that case.
+	 */
+	if (am_walsender
+		|| (!(MyProc->statusFlags & PROC_IN_SAFE_IC)
+			&& !TransactionIdIsValid(GetCurrentTransactionIdIfAny())))
 		return retry;
 
 	/* Calculate replicas lag */