IPv6-only clients fetch microdescriptors from a small number of IPv6 fallbacks

When an IPv6-only client bootstraps using microdescriptors (legacy/trac#19608 (moved)), it fetches the microdescriptor consensus from an IPv6 fallback, but the microdescriptor consensus has no IPv6 addresses.

So it falls back to the fallback directories, fetching ~7500/500 = 15 sets of descriptors from 15 of the 25 IPv6 fallbacks.

We should improve this behaviour somehow, to avoid overloading the fallbacks. One simple way of doing this is selecting 200 fallbacks for 0.2.9 in legacy/trac#18828 (moved).

It's worth noting that this extra load only happens on bootstrap, when there are no cached microdescriptors. If an IPv6-only client has any IPv6 microdescriptors that match the current consensus, it will use those relays instead.