Make StringDecoder use DataBufferUtils.split

* Added DataBufferUtils.split variant that takes multiple delimiters
  as argument (instead of 1).
* Use this new split() variant from within StringDecoder, replacing
  its inefficient algorithm with the Knuth-Morris-Pratt algorithm.

spring-core/src/test/java/org/springframework/core/io/buffer/DataBufferUtilsTests.java

@@ -820,6 +820,25 @@ public class DataBufferUtilsTests extends AbstractDataBufferAllocatingTestCase {
 				.verifyComplete();
 	}
 
+	@Test
+	public void splitMultipleDelimiters() {
+		Mono<DataBuffer> source =
+				deferStringBuffer("foo␤bar␍␤baz␤");
+
+		byte[][] delimiters = new byte[][]{
+				"␤".getBytes(StandardCharsets.UTF_8),
+				"␍␤".getBytes(StandardCharsets.UTF_8)
+		};
+
+		Flux<DataBuffer> result = DataBufferUtils.split(source, delimiters, false);
+
+		StepVerifier.create(result)
+				.consumeNextWith(stringConsumer("foo␤"))
+				.consumeNextWith(stringConsumer("bar␍␤"))
+				.consumeNextWith(stringConsumer("baz␤"))
+				.verifyComplete();
+	}
+
 	@Test
 	public void splitErrors() {
 		Flux<DataBuffer> source = Flux.concat(