Make SNI per-connection, not per-session.

Right now we report the per-connection value during the handshake and
the per-session value after the handshake. This also trims our tickets
slightly by removing a largely unused field from SSL_SESSION.

Putting it on SSL_HANDSHAKE would be better, but sadly a number of
bindings-type APIs expose it after the handshake.

Change-Id: I6a1383f95da9b1b141b9d6adadc05ee1e458a326
Reviewed-on: https://boringssl-review.googlesource.com/20064
Commit-Queue: David Benjamin <davidben@google.com>
Commit-Queue: Steven Valdez <svaldez@google.com>
Reviewed-by: Steven Valdez <svaldez@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

ssl/ssl_lib.cc

diff --git a/ssl/ssl_lib.cc b/ssl/ssl_lib.cc
index 853994b..7a75776 100644
--- a/ssl/ssl_lib.cc
+++ b/ssl/ssl_lib.cc
@@ -1755,25 +1755,11 @@
     return ssl->tlsext_hostname;
   }
 
-  // During the handshake, report the handshake value.
-  if (ssl->s3->hs != NULL) {
-    return ssl->s3->hs->hostname.get();
-  }
-
-  // SSL_get_servername may also be called after the handshake to look up the
-  // SNI value.
-  //
-  // TODO(davidben): This is almost unused. Can we remove it?
-  SSL_SESSION *session = SSL_get_session(ssl);
-  if (session == NULL) {
-    return NULL;
-  }
-  return session->tlsext_hostname;
+  return ssl->s3->hostname;
 }
 
 int SSL_get_servername_type(const SSL *ssl) {
-  SSL_SESSION *session = SSL_get_session(ssl);
-  if (session == NULL || session->tlsext_hostname == NULL) {
+  if (SSL_get_servername(ssl, TLSEXT_NAMETYPE_host_name) == NULL) {
     return -1;
   }
   return TLSEXT_NAMETYPE_host_name;