Current code takes a reference and holds it past preemption points. And
while the state itself is not suppose to change the reference may
become stale because the state is re-created on each raft topology
command.
Fix it by taking a copy instead. This is a slow path anyway.
Fixes: scylladb/scylladb#21220
(cherry picked from commit fb38bfa35d)
Closesscylladb/scylladb#21372