add the "explicit-exit-notify" option as a default for OpenVPN Server instances
Suppose you have a UDP OpenVPN connection between "Client" and "Server".
If Client is restarted, because it is a client, it will renegotiate new
TLS keys with the server, and the keys will immediately be in sync.
However if the Server is restarted, it will not know that it must force a
key renegotiation with a previously connected client because servers don't
initiate connections, they wait for them to be initiated by clients.