Load SLRU segments on demand

[knizhnik]

refer https://github.com/neondatabase/cloud/issues/8673

[hlinnaka]

Do we also need to do on-demand download in SlruPhysicalWritePage()? If not, why not?

[hlinnaka]

Suggested change

	/* If page is greather than latest written page, then do not try to download segment from server */
	/* If page is greater than latest written page, then do not try to download segment from server */

[knizhnik]

Done

[knizhnik]

Do we also need to do on-demand download in SlruPhysicalWritePage()? If not, why not?

Before writing page Postgres first cake if it exists:

	/* Create the current segment file, if necessary */
	if (!SimpleLruDoesPhysicalPageExist(CommitTsCtl, pageno))
	{
		int			slotno;

		LWLockAcquire(CommitTsSLRULock, LW_EXCLUSIVE);
		slotno = ZeroCommitTsPage(pageno, false);
		SimpleLruWritePage(CommitTsCtl, slotno);
		Assert(!CommitTsCtl->shared->page_dirty[slotno]);
		LWLockRelease(CommitTsSLRULock);
	}

and SimpleLruDoesPhysicalPageExist tries to load page from PS if it doesn't exists.

There are few more cases in slru.c where SlruInternalWritePage is called (except SimpleLruWritePage) but it is done for pages present in the slot and they should be checked and downloaded before.