tor-hscrypto: Return 0 if the timestamp is before the start of the TP.
#1155 (closed) was happening because we couldn't compute the offset of the
current time from the start of the next TP
(TimePeriod::offset_within_period
expected when
to come after the
start of the TP). TimePeriod::offset_within_period
now returns an
offset of 0 for timestamps that come before the start of the TP, to
support computing revision counters for the descriptors uploaded to
the HsDirs from the ring associated with the next TP.
Fixes #1155 (closed)