Remember last written LSN when it is first requested (#7343)

## Problem

See https://neondb.slack.com/archives/C03QLRH7PPD/p1712529369520409

In case of statements CREATE TABLE AS SELECT... or INSERT FROM SELECT...
we are fetching data from source table and storing it in destination
table. It cause problems with prefetch last-written-lsn is known for the
pages of source table
(which for example happens after compute restart). In this case we get
get global value of last-written-lsn which is changed frequently as far
as we are writing pages of destination table. As a result request-isn
for the prefetch and request-let when this page is actually needed are
different and we got exported prefetch request. So it actually disarms
prefetch.


## Summary of changes

Proposed simple patch stores last-written LSN for the page when it is
not found. So next time we will request last-written LSN for this page,
we will get the same value (certainly if the page was not changed).

## 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>

vendor/revisions.json

@@ -1,5 +1,5 @@
 {
-  "postgres-v16": "3946b2e2ea71d07af092099cb5bcae76a69b90d6",
-  "postgres-v15": "64b8c7bccc6b77e04795e2d4cf6ad82dc8d987ed",
-  "postgres-v14": "a7b4c66156bce00afa60e5592d4284ba9e40b4cf"
+  "postgres-v16": "261497dd63ace434045058b1453bcbaaa83f23e5",
+  "postgres-v15": "85d809c124a898847a97d66a211f7d5ef4f8e0cb",
+  "postgres-v14": "d9149dc59abcbeeb26293707509aef51752db28f"
 }