tor-netdir: Add separate functions for computing hsdirs for upload/download.
This is an alternative to !1514 (closed)
The hsdir selection algorithm for uploads and downloads is different
enough to justify splitting hs_dirs into 2 different functions.
More specifically, when selecting the relays to upload a service's
descriptors to, the service's hsids need to be matched up with the
correct ring (using the time period) before applying select_nodes to
pick the replicas. This is not the case when downloading, because
for downloads select relays from the current ring.
Edited by gabi-250