crimson/mon: do not set_value() again

crimson/mon: do not set_value() again

once the continuation consuming the stored value of the associated
future, we cannot set_value() again. otherwise, ASan complains that we
are accessing the memory on heap after it is freed.

in this change, std::optional<> is used for holding
promise<auth_result_t>, once the promise is fulfilled, `auth_done` is
reset to prevent another call of `set_value()` or `set_exception()`.

Signed-off-by: Kefu Chai <kchai@redhat.com>