Server adapters release buffers on error/cancel

Review and update Servlet and Undertow adapters to release any data
buffers they be holding on to at the time of error or cancellation.

Also remove onDiscard hooks from Reactor and Undertow request body.
For Reactor we expect it to be handled. For Undertow there isn't
any Reactor Core upstream for the callback to be useful.

Issue: SPR-17410

spring-webflux/src/main/java/org/springframework/web/reactive/socket/adapter/AbstractListenerWebSocketSession.java

@@ -260,11 +260,23 @@ public abstract class AbstractListenerWebSocketSession<T> extends AbstractWebSoc
 				rsReadLogger.trace(getLogPrefix() + "Received " + message);
 			}
 			if (!this.pendingMessages.offer(message)) {
+				discardData();
 				throw new IllegalStateException(
 						"Too many messages. Please ensure WebSocketSession.receive() is subscribed to.");
 			}
 			onDataAvailable();
 		}
+
+		@Override
+		protected void discardData() {
+			while (true) {
+				WebSocketMessage message = (WebSocketMessage) this.pendingMessages.poll();
+				if (message == null) {
+					return;
+				}
+				message.release();
+			}
+		}
 	}
 
 
@@ -289,6 +301,7 @@ public abstract class AbstractListenerWebSocketSession<T> extends AbstractWebSoc
 			else if (rsWriteLogger.isTraceEnabled()) {
 				rsWriteLogger.trace(getLogPrefix() + "Sending " + message);
 			}
+			// In case of IOException, onError handling should call discardData(WebSocketMessage)..
 			return sendMessage(message);
 		}
 
@@ -313,6 +326,11 @@ public abstract class AbstractListenerWebSocketSession<T> extends AbstractWebSoc
 			}
 			this.isReady = ready;
 		}
+
+		@Override
+		protected void discardData(WebSocketMessage message) {
+			message.release();
+		}
 	}
 
 }