Export server-side ticket_age skew.

We'll measure this value to guide what tolerance to use in the 0-RTT
anti-replay mechanism. This also fixes a bug where we were previously
minting ticket_age_add-less tickets on the server. Add a check to reject
all those tickets.

BUG=113

Change-Id: I68e690c0794234234e0d0500b4b9a7f79aea641e
Reviewed-on: https://boringssl-review.googlesource.com/14068
Reviewed-by: Steven Valdez <svaldez@google.com>
Commit-Queue: Steven Valdez <svaldez@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

ssl/ssl_lib.c

diff --git a/ssl/ssl_lib.c b/ssl/ssl_lib.c
index 856fba2..b207e92 100644
--- a/ssl/ssl_lib.c
+++ b/ssl/ssl_lib.c
@@ -2557,6 +2557,10 @@
   ctx->grease_enabled = !!enabled;
 }
 
+int32_t SSL_get_ticket_age_skew(const SSL *ssl) {
+  return ssl->s3->ticket_age_skew;
+}
+
 int SSL_clear(SSL *ssl) {
   /* In OpenSSL, reusing a client |SSL| with |SSL_clear| causes the previously
    * established session to be offered the next time around. wpa_supplicant