Don't overestimate the ticket age

On the client we calculate the age of the ticket in seconds but the server
may work in ms. Due to rounding errors we could overestimate the age by up
to 1s. It is better to underestimate it. Otherwise, if the RTT is very
short, when the server calculates the age reported by the client it could
be bigger than the age calculated on the server - which should never happen.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5306)