Refs MW-252

- enveloped FTWRL processing with wsrep desync/resync calls. This way FTWRL processing node
  will not cause flow control to kick in
- donor servicing thread is unfortunate exception, we must let him to pause provider as part
  of FTWRL phase, but not desync/resync as this is done as part of donor control on higher
  level

sql/sql_class.h

@@ -3880,6 +3880,7 @@ public:
   bool                      wsrep_apply_toi; /* applier processing in TOI */
   bool                      wsrep_skip_append_keys;
   wsrep_gtid_t              wsrep_sync_wait_gtid;
+  my_bool                   wsrep_donor; /* true if thread is SST donor servicing */
   ulong                     wsrep_affected_rows;
 #endif /* WITH_WSREP */
 };