Server does not seem to support season ticket sealer in TLS 1.2

While debugging a server using tris, I was very confused by errors relating to session tickets: it seems there were panics due to missing keys. Contrasting https://github.com/cloudflare/tls-tris/blob/master/13.go#L829 with https://github.com/cloudflare/tls-tris/blob/6e853ddcae84e1cbe1f2d843c1597843e804a02d/handshake_server.go#L725 and https://github.com/cloudflare/tls-tris/blob/master/handshake_server.go#L102 it's clear what the problem is. Patch will be incoming shortly.

Compare the TLS 1.3 logic in 13.go at line 829 with the TLS 1.2 paths in handshake_server.go at lines 725 and 102. Trace how the season ticket sealer and its keys are handled, then verify that a TLS 1.2 server no longer panics because the keys are missing.

Written by the indexing model from the issue text.