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.

src/wsrep_provider_v26.cpp

@@ -895,6 +895,7 @@ enum wsrep::provider::status
 wsrep::wsrep_provider_v26::bf_abort(
     wsrep::seqno bf_seqno,
     wsrep::transaction_id victim_id,
+    wsrep::operation_context& /* Ignored here */,
     wsrep::seqno& victim_seqno)
 {
     wsrep_seqno_t wsrep_victim_seqno;