Allow up to clock_step error when checking the session timestamps
Allow up to clock_step error when checking the session timestamps

When checking the timestamps on deleting a session, allow up to
clock_step seconds error before bailing.

Fixes: #1874

gquintard/Varnish-Cache,feld/Varnish-Cache,feld/Varnish-Cache,gquintard/Varnish-Cache,feld/Varnish-Cache,feld/Varnish-Cache,feld/Varnish-Cache,gquintard/Varnish-Cache,gquintard/Varnish-Cache