Add API to query the current onion key grace period.

This patch adds an API to get the current grace period, in days, defined
as the consensus parameter "onion-key-grace-period-days".

As per proposal #274 the values for "onion-key-grace-period-days" is a
default value of 7 days, a minimum value of 1 day, and a maximum value
defined by other consensus parameter "onion-key-rotation-days" also
defined in days.

See: https://bugs.torproject.org/21641