DTLS handshake fix.

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

Fix handling of application data received before a handshake.
(cherry picked from commit 0c75eeacd3285b395dc75b65c3e6fe6ffbef59f0)
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.
(cherry picked from commit b62f4daac00303280361924b9cc19b3e27528b15)