Do not retain cache transaction managers

Previously, cache transaction managers may be  retained outside the
boundaries of an application context with AspectJ since an aspect is
basically a singleton for the current class loader.

This commit adds a "clearTransactionManagerCache" that is similar to the
"clearMetadataCache" introduced in CacheAspectSupport: whenever the
context is disposed, the cache is cleared to remove any reference to a
transaction manager defined by that context.

Issue: SPR-12518

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

@@ -338,6 +338,13 @@ public abstract class TransactionAspectSupport implements BeanFactoryAware, Init
 		}
 	}
 
+	/**
+	 * Clear the cached transaction managers.
+	 */
+	protected void clearTransactionManagerCache() {
+		this.transactionManagerCache.clear();
+	}
+
 	/**
 	 * Determine the specific transaction manager to use for the given transaction.
 	 */