This commits add some new options to examples/[server,client] to support testing
of DTLS v1.3.
client: add waitTicket option
If this option is used, the client will wait until it receives a sessionTicket
from the server. This is useful when testing DTLS retransmission.
client: add waitKeyUpdate option
When this option is set, the client waits until the UpdateKey message is
acknowledged by the server. This is useful to test DTLS retransmission logic
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