(Share) Lock buffer page when reading from columnar storage (#5338)

Under high write concurrency, we were sometimes reading columnar
metapage as all zeros.

In `WriteToBlock()`, if `clear == true`, then it will clear the page before
writing the new one, rather than just adding data to the page. That
means any concurrent connection that is holding only a pin will be
able to see the all-zero state between the `InitPage()` and the
`memcpy_s()`.

Moreover, postgres/storage/buffer/README states that:

> Buffer access rules:
>
> 1. To scan a page for tuples, one must hold a pin and either shared or
> exclusive content lock.  To examine the commit status (XIDs and status bits)
> of a tuple in a shared buffer, one must likewise hold a pin and either shared
> or exclusive lock.

For those reasons, we have to make sure to never keep a pin on the
page without (at least) the shared lock, to avoid having such problems.

src/backend/columnar/columnar_storage.c

@@ -667,6 +667,7 @@ ReadFromBlock(Relation rel, BlockNumber blockno, uint32 offset, char *buf,
 			  uint32 len, bool force)
 {
 	Buffer buffer = ReadBuffer(rel, blockno);
+	LockBuffer(buffer, BUFFER_LOCK_SHARE);
 	Page page = BufferGetPage(buffer);
 	PageHeader phdr = (PageHeader) page;
 
@@ -678,7 +679,7 @@ ReadFromBlock(Relation rel, BlockNumber blockno, uint32 offset, char *buf,
 	}
 
 	memcpy_s(buf, len, page + offset, len);
-	ReleaseBuffer(buffer);
+	UnlockReleaseBuffer(buffer);
 }