MDEV-20848 Fixes for MTR test galera_sr.GCF-1060

[sciascid]

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.

[claassistantio]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}