Don't deadlock kill event groups in other domains if they are not
SPECULATE_OPTIMISTIC. Such event groups may not be able to safely roll back
and retry (eg. DDL).

But do deadlock kill a transaction T2 from a blocked transaction U in another
domain, even if T2 has lower sub_id than U. Otherwise, in case of a cycle
T2->T1->U->T2, we might not break the cycle if U is not SPECULATE_OPTIMISTIC
Signed-off-by: Kristian Nielsen <knielsen@knielsen-hq.org>