add OnUpgrade callback

[kokizzu]

related to this: coredns/coredns#6074

so all plugin that require releasing port can add themself to this callback.

[chrisohaver]

Maybe instead of adding a new callback, call existing OnShutdown callbacks during Upgrade?

[kokizzu]

Just not sure if this will affect every plugin and giving unexpected behavior instead of just plugins that require closing socket

[kokizzu]

yup, can't do, if i call shutdown before upgrade, it would change a lot of plugins behavior because normally OnShutdown called after new forked process is up, so it's better to call different event (OnUpgrade) instead (or maybe name this OnBeforeUpgrade?)