It seems that returning 0 when enc == 0 crashes OpenSSL.

This option specifies files contains 48 random bytes to construct
session ticket key data.  This option can be used repeatedly to
specify multiple keys, but only the first one is used to encrypt
tickets.

On reception of GOAWAY, new stream creation is disallowed regardless
of last-stream-id in GOAWAY is larger than next stream ID.

Update is done by main event loop which is stopped after graceful
shutdown is commenced, which means time is no longer update.  To avoid
this situation, we just avoid caching and get time for each logging.

This is because std::chrono::high_resolution_clock may not be wall
clock; it may be alias of std::chrono::stead_clock.