This patch contains two fixes:

* wsrep_handle_mdl_conflict(): handle the case where SR transaction
  is in aborting state. Previously, a BF-BF conflict was reported, and
  the process would abort.
* wsrep_thd_bf_abort(): do not restore thread vars after calling
  wsrep_bf_abort(). Thread vars are already restored in wsrep-lib if
  necessary. This also removes the assumption that the caller of
  wsrep_thd_bf_abort() is the given bf_thd, which is not the case.

Also in this patch:

* Remove unnecessary check for active victim transaction in
  wsrep_thd_bf_abort(): the exact same check is performed later in
  wsrep_bf_abort().
* Make wsrep_thd_bf_abort() and wsrep_log_thd() const-correct.
* Change signature of wsrep_abort_thd() to take THD pointers instead
  of void pointers.