Use LW_SHARED in walsummarizer.c for WALSummarizerLock lock where possible.

Previously, we used LW_EXCLUSIVE in several places despite only reading
WalSummarizerCtl fields. This patch reduces the lock level to LW_SHARED
where we are only reading the shared fields.

Backpatch to 17, where wal summarization was introduced.

Reviewed-by: Nathan Bossart <nathandbossart@gmail.com>
Discussion: https://postgr.es/m/CAD21AoDdKhf_9oriEYxY-JCdF+Oe_muhca3pcdkMEdBMzyHyKw@mail.gmail.com
Backpatch-through: 17

src/backend/postmaster/walsummarizer.c

@@ -641,7 +641,7 @@ SetWalSummarizerLatch(void)
 	if (WalSummarizerCtl == NULL)
 		return;
 
-	LWLockAcquire(WALSummarizerLock, LW_EXCLUSIVE);
+	LWLockAcquire(WALSummarizerLock, LW_SHARED);
 	pgprocno = WalSummarizerCtl->summarizer_pgprocno;
 	LWLockRelease(WALSummarizerLock);
 
@@ -682,7 +682,7 @@ WaitForWalSummarization(XLogRecPtr lsn)
 		/*
 		 * If the LSN summarized on disk has reached the target value, stop.
 		 */
-		LWLockAcquire(WALSummarizerLock, LW_EXCLUSIVE);
+		LWLockAcquire(WALSummarizerLock, LW_SHARED);
 		summarized_lsn = WalSummarizerCtl->summarized_lsn;
 		pending_lsn = WalSummarizerCtl->pending_lsn;
 		LWLockRelease(WALSummarizerLock);