Allow ConfigurationCPP to process multiple registries

Prior to this change, an instance of ConfigurationClassPostProcessor
would throw IllegalStateException if its
postProcessBeanDefinitionRegistry method were called more than once.
This check is important to ensure that @Configuration classes are
not proxied by CGLIB multiple times, and works for most normal use
cases.

However, if the same CCPP instance is used to process multiple
registries/factories/contexts, this check creates a false negative
because it does not distinguish between invocations of
postProcessBeanDefinitionRegistry across different registries.

A use case for this, though admittedly uncommon, would be creating
a CCPP instance and registering it via
ConfigurableApplicationContext#addBeanDefinitionPostProcessor against
several ApplicationContexts. In such a case, the same CCPP instance
will post-process multiple different registry instances, and throw the
above mentioned exception.

With this change, CCPP now performs lightweight tracking of the
registries/beanFactories that it has already processed by recording
the identity hashcodes of these objects.  This is only slightly more
complex than the previous boolean-based 'already processed' flags, and
prevents this issue (however rare it may be) from occurring.

Issue: SPR-8527

org.springframework.context/src/test/java/org/springframework/context/annotation/ConfigurationClassPostProcessorTests.java

@@ -82,6 +82,25 @@ public class ConfigurationClassPostProcessorTests {
 		assertSame(foo, bar.foo);
 	}
 
+	@Test
+	public void testProcessingAllowedOnlyOncePerProcessorRegistryPair() {
+		DefaultListableBeanFactory bf1 = new DefaultListableBeanFactory();
+		DefaultListableBeanFactory bf2 = new DefaultListableBeanFactory();
+		ConfigurationClassPostProcessor pp = new ConfigurationClassPostProcessor();
+		pp.postProcessBeanFactory(bf1); // first invocation -- should succeed
+		try {
+			pp.postProcessBeanFactory(bf1); // second invocation for bf1 -- should throw
+			fail("expected exception");
+		} catch (IllegalStateException ex) {
+		}
+		pp.postProcessBeanFactory(bf2); // first invocation for bf2 -- should succeed
+		try {
+			pp.postProcessBeanFactory(bf2); // second invocation for bf2 -- should throw
+			fail("expected exception");
+		} catch (IllegalStateException ex) {
+		}
+	}
+
 
 	@Configuration
 	static class SingletonBeanConfig {