Modify clients to consistently send N% of initial consensus requests to the authorities

We currently use the directory authority statistics to estimate the number of clients in the Tor network.

But fallback directories will handle initial consensus downloads in future releases.

So, to preserve client statistics, we can modify Tor clients to make a certain small percentage (1%?) of initial consensus downloads to the directory authorities. If we scale the authority downloads from each Tor release by this factor, we can estimate the number of clients.

This is particularly important for client IPv6 bootstrap (legacy/trac#17840 (moved)), as only some authorities and fallbacks have IPv6 (and the proportion of fallbacks is lower than the proportion of authorities).

We could change the meaning of DirAuthorityFallbackRate to "the authorities see this proportion of queries", or implement another option, and document the interaction with DirAuthorityFallbackRate.

(I think the second option is better, but we'd end up essentially obsoleting DirAuthorityFallbackRate.)