DTLS handshake fix.

Reported by: Prashant Jaikumar <rmstar@gmail.com>

Fix handling of application data received before a handshake.
DTLS message_sequence number wrong in rehandshake ServerHello

This fix ensures that
* A HelloRequest is retransmitted if not responded by a ClientHello
* The HelloRequest "consumes" the sequence number 0. The subsequent
ServerHello uses the sequence number 1.
* The client also expects the sequence number of the ServerHello to
be 1 if a HelloRequest was received earlier.
This patch fixes the RFC violation.