Add pending state to sensor

[jayakornk]

• Fix issues:
  • https://github.com/meichthys/uptime_kuma/issues/6
  • https://github.com/meichthys/uptime_kuma/issues/7
  • https://github.com/meichthys/uptime_kuma/issues/8

I think the main problem is sensor switch to DOWN when retrying.

[meichthys]

Thanks for taking a look at this!