common/ceph_timer: Pass reference to waited time on stack

common/ceph_timer: Pass reference to waited time on stack

std::condition_variable::wait_until takes a const reference to a
time_point. It may access this reference after relinquishing the
mutex, creating a potential use-after-free error if the first event is
shut down.

So, just copy the time onto the stack, so we have a reference that
won't disappear.

https://tracker.ceph.com/issues/44373

Signed-off-by: Adam C. Emerson <aemerson@redhat.com>