Optimize for single TransactionSynchronization

Optimize TransactionSynchronizationManager.getSynchronizations() to
avoid sorting and ArrayList creation in case of a single
TransactionSynchronization.

See gh-27335

spring-tx/src/main/java/org/springframework/transaction/support/TransactionSynchronizationManager.java

@@ -297,6 +297,9 @@ public abstract class TransactionSynchronizationManager {
 		if (synchs.isEmpty()) {
 			return Collections.emptyList();
 		}
+		else if (synchs.size() == 1) {
+			return Collections.singletonList(synchs.iterator().next());
+		}
 		else {
 			// Sort lazily here, not in registerSynchronization.
 			List<TransactionSynchronization> sortedSynchs = new ArrayList<>(synchs);