HsCircPool should discard all circuits when we switch between full and lite vanguards
In !2035 (merged), the vanguards API was designed to purge all HS circuits whenever the vanguard "mode" changed. However, that design assumed the vanguard mode (full/lite/disabled) would be read from the config, which is no longer true: as of !2083 (merged), we now have 2 vanguard "modes", both of which are obtained from the consensus. The two modes, along with a boolean flag that specifies whether we're running an onion service or not, are used to compute the "mode" the vanguard manager is supposed to operate in.
The mode is computed on-the-fly, so we can't easily detect when it changes. Moreover, it means we can't simply discard circuits in response to configuration changes: we need to take the two "modes" specified in the consensus into account too! In other words, the current mechanism for discarding HS circuits is broken.