This fix is the refactoring of the abandon operation (issue #1885).

- AbandonOperation is now an interface
- AbandonOperationBasis implements all the processing

There is no workflow element for the abandon operation because
the abandon operation has no request base DN hence the operation
cannot be routed to a workflow. This is harmless since all the processing
of the abandon operation is generic (search for the operation to abandon
and do a cancel() on the abandonned operation). As a consequence,  the
PostOperationAbandonOperation plugin interface is implemented by the
global object AbandonOperationBasis.