Pass victim context for provider on BF abort

This change is needed for custom provider implementations to
have a way to access the victim in the application context.

Helper interface operation_context to pass caller context for
service/provider callbacks in more type safe way.

test/mock_provider.hpp

@@ -310,6 +310,7 @@ namespace wsrep
         enum wsrep::provider::status
         bf_abort(wsrep::seqno bf_seqno,
                  wsrep::transaction_id trx_id,
+                 wsrep::operation_context&,
                  wsrep::seqno& victim_seqno)
             WSREP_OVERRIDE
         {