Add application defined sequential consistency for certification

Client state methods before_prepare() and before_commit() accept
a callback which is called by the provider after it can guarantee
sequential consistency. This allows application threads which wish to
maintain sequential consistency to enter before_prepare() and
before_commit() calls concurrently without waiting the prior call
to finish.

test/mock_provider.hpp

@@ -79,7 +79,8 @@ namespace wsrep
         certify(wsrep::client_id client_id,
                 wsrep::ws_handle& ws_handle,
                 int flags,
-                wsrep::ws_meta& ws_meta)
+                wsrep::ws_meta& ws_meta,
+                const seq_cb* /* Ignored in unit tests. */)
             WSREP_OVERRIDE
         {
             ws_handle = wsrep::ws_handle(ws_handle.transaction_id(), (void*)1);