INT-2815 Refactor AbstractPollingEndpoint

Recent changes to APE were sub-optimal because the subclasses
had to call back into the parent abstract class.

Deprecate doPoll() on APE.

Introduce a temporary intermediate abstract class that supports
transaction synchronization.

This class will be deprecated in 3.0.0 when its methods will
be pulled up into APE.

INT-2815 Polishing

Make ATSPE package visibility.

Add helpers to enable early migration to 3.0 structure.

INT-2815 Polishing

Fix javadoc to emphasize that doPoll() will no longer
be overridable in 3.0.

Rename doReceive() to receiveMessage() to match more closely
with handleMessage().

INT-2815 Polishing

Fix message in UnsupportedOperationException.

spring-integration-core/src/test/java/org/springframework/integration/endpoint/PollingEndpointStub.java

@@ -16,7 +16,6 @@
 
 package org.springframework.integration.endpoint;
 
-import org.springframework.integration.Message;
 import org.springframework.scheduling.support.PeriodicTrigger;
 
 /**
@@ -33,19 +32,4 @@ public class PollingEndpointStub extends AbstractPollingEndpoint {
 		throw new RuntimeException("intentional test failure");
 	}
 
-	@Override
-	protected Message<?> doReceive() {
-		return null;
-	}
-
-	@Override
-	protected Object getResourceToBind() {
-		return null;
-	}
-
-	@Override
-	protected String getResourceKey() {
-		return null;
-	}
-
 }