crypto/tls: add SessionState and use it on the server side

crypto/tls: add SessionState and use it on the server side

This change by itself is useless, because the application has no way to
access or provide SessionStates to crypto/tls, but they will be provided
in following CLs.

For #60105

Change-Id: I8d5de79b1eda0a778420134cf6f346246a1bb296
Reviewed-on: https://go-review.googlesource.com/c/go/+/496818
Reviewed-by: Marten Seemann <martenseemann@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Filippo Valsorda <filippo@golang.org>