This is a backport of the applicable part of
commit 93475aff and
commit 2c39f69d
from 10.4.

Before 10.4 and Galera 4, WSREP_ON is a macro that points to
a global Boolean variable, so it is not that expensive to
evaluate, but we will add an unlikely() hint around it.

WSREP_ON_NEW: Remove. This macro was introduced in
commit c863159c
when reverting WSREP_ON to its previous definition.

We replace some use of WSREP_ON with WSREP(thd), like it was done
in 93475aff. Note: the macro
WSREP() in 10.1 is equivalent to WSREP_NNULL() in 10.4.

Item_func_rand::seed_random(): Avoid invoking current_thd
when WSREP is not enabled.