Calculate responsible HSDirs for v3 descriptors in stem
The v3 HSPOST command will only post the current descriptor if we don't explicitly provide the HSDirs, but we also need to post the next descriptor from onionbalance.
This means that we will need to compute the right HSDirs for both current and next descriptors from within stem.
Furthermore, we will need to implement the whole time-period/SRV logic on the onionbalance side to know which time-period/SRV to use at any given time, and also when a new consensus arrives.