INT-3899: Defer Poller Creation Until start()

JIRA: https://jira.spring.io/browse/INT-3899

Previously, within `AbstractPollingEndpoint`, the `Poller` object was
created from `afterPropertiesSet()` as well as `start()` if previously `stopped()`
(`this.initialized == false`).

This precluded modification of the advice chain (e.g. in tests when using
`autoStartup == false`).

Move the poller creation to `start()` so that it is always created then, and does
not require a previous `stop()` to change the advice chain.

spring-integration-ftp/src/test/java/org/springframework/integration/ftp/config/FtpInboundChannelAdapterParserTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2014 the original author or authors.
+ * Copyright 2002-2016 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -90,7 +90,6 @@ public class FtpInboundChannelAdapterParserTests {
 		assertNotNull(comparator);
 		assertEquals("ftpInbound", ftpInbound.getComponentName());
 		assertEquals("ftp:inbound-channel-adapter", ftpInbound.getComponentType());
-		assertNotNull(TestUtils.getPropertyValue(ftpInbound, "poller"));
 		assertEquals(context.getBean("ftpChannel"), TestUtils.getPropertyValue(ftpInbound, "outputChannel"));
 		FtpInboundFileSynchronizingMessageSource inbound =
 			(FtpInboundFileSynchronizingMessageSource) TestUtils.getPropertyValue(ftpInbound, "source");