make RetryableException a marker interface

lib/Doctrine/DBAL/Exception/DeadlockException.php

@@ -26,6 +26,6 @@ namespace Doctrine\DBAL\Exception;
  * @link   www.doctrine-project.org
  * @since  2.6
  */
-class DeadlockException extends RetryableException
+class DeadlockException extends ServerException implements RetryableException
 {
 }

lib/Doctrine/DBAL/Exception/LockWaitTimeoutException.php

@@ -26,6 +26,6 @@ namespace Doctrine\DBAL\Exception;
  * @link   www.doctrine-project.org
  * @since  2.6
  */
-class LockWaitTimeoutException extends RetryableException
+class LockWaitTimeoutException extends ServerException implements RetryableException
 {
 }

lib/Doctrine/DBAL/Exception/RetryableException.php

@@ -20,12 +20,12 @@
 namespace Doctrine\DBAL\Exception;
 
 /**
- * Base class for all exceptions where retrying the transaction makes sense.
+ * Marker interface for all exceptions where retrying the transaction makes sense.
  *
  * @author Tobias Schultze <http://tobion.de>
  * @link   www.doctrine-project.org
  * @since  2.6
  */
-abstract class RetryableException extends ServerException
+interface RetryableException
 {
 }

lib/Doctrine/DBAL/RetryWrapper.php

@@ -102,8 +102,9 @@ class RetryWrapper
      * The callable is only re-executed for temporary database errors where retrying
      * the failed transaction after a short delay usually resolves the problem. Such
      * errors are for example deadlocks and lock wait timeouts. Internally the raised
-     * exception must extend RetryableException. Other exceptions like syntax errors
-     * or constraint violations will not cause the callable to be re-executed.
+     * exception must implement the marker interface RetryableException. Other exceptions
+     * like syntax errors or constraint violations will not cause the callable to be
+     * re-executed.
      *
      * All arguments given will be passed through to the wrapped callable.
      *