Periodically republish the descriptor

The descriptor publisher needs to implement this part of the spec:

Specifically, every time a hidden service publishes its descriptor, it also sets up a timer for a random time
between 60 minutes and 120 minutes in the future. When the timer triggers, the hidden service needs to 
publish its descriptor again to the responsible HSDirs for that time period. [TODO: Control republish period 
using a consensus parameter?]