Do not retain BeanFactory ref

This is a rework of fd7153ff that also now clears the reference to the
BeanFactory when the context is disposed.

Issue: SPR-12518

spring-tx/src/main/java/org/springframework/transaction/interceptor/TransactionAspectSupport.java

@@ -339,10 +339,11 @@ public abstract class TransactionAspectSupport implements BeanFactoryAware, Init
 	}
 
 	/**
-	 * Clear the cached transaction managers.
+	 * Clear the cache.
 	 */
 	protected void clearTransactionManagerCache() {
 		this.transactionManagerCache.clear();
+		this.beanFactory = null;
 	}
 
 	/**