[tls] Parse ServerKeyExchange record immediately

[tls] Parse ServerKeyExchange record immediately

As of commit 433a8f5 ("[tls] Retain a reference in the key schedule to
the bound identity"), the act of binding the server identity is
logically separated from the act of validating the server identity.
We may therefore bind the server identity (by verifying the signature
over the Diffie-Hellman parameters) and agree the ephemeral shared
secret immediately upon receiving the ServerKeyExchange record, rather
than deferring the verification until we have a validated identity.

This provides a closer match to the flow required for TLS version 1.3,
where the ephemeral shared secret is used for all messages after
ServerHello, and so must always be agreed prior to validation.

Signed-off-by: Michael Brown <mcb30@ipxe.org>