Fix DTLS handshake infinite loop

[NicFontana]

Fix DTLS handshake infinite loop caused by the client dropping packets different from Client Hello kind of packets.

This happens when it receives the Hello Verify Request after its resend timeout (initially set to 1000ms) expires.

Fix #726

[peterdettman]

I think you've correctly identified readVersion handling as a problem, but it might be more straightforward to just not set it from HelloVerifyRequest recordVersion. The current code clears readVersion after it receives a HelloVerifyRequest message, but that clearing doesn't happen for retransmitted records.

[peterdettman]

I've fixed this by not setting readVersion from HelloVerifyRequest instead, but we have credited you with the initial fix.