Use a reasonable record version for close_notify sent before version negotiation

[raycoll]

Problem:

If an application acting as a server calls s2n_shutdown before it receives a complete ClientHello handshake message from the client, s2n will send a close_notify alert record with record version 0x00, 0x00. Receivers are unlikely to handle to handle this record correctly and may mischaractize the reason for connection closure(Closure exception vs parsing exception).

Proposed Solution:

Pick a default record version for records sent before versions are negotiated. Since TLS 1.3 freezes record version to be 0x0303, I recommend using that.

[raycoll]

one could argue that an application shouldn't call s2n_shutdown before version negotiation is complete, however the spec is not clear here