tor-hsservice: Replace {current,previous}_period with a list of time periods.

The descriptor publisher reactor was (incorrectly) only keeping track of the "current" and "previous" time periods.

The reactor now maintains a list of all relevant time periods (obtained from Netdir::hs_all_time_periods).

This MR also addresses a deferred review comment !1545 (comment 2935668) (in gabi-250/arti@a63652df).