Write and analyze proposals for fetching consensuses/microdescriptors less frequently?

The idea: Our current algorithm for deciding whether you need a new consensus is ad hoc; we just picked an interval more or less at random.

Depending on the results from legacy/trac#21205 (moved), we may learn that it's not as necessary as we had thought for a client to fetch consensuses and microdescriptors so often. If that's the case, we should have proposals and analyses for (optionally?) decreasing the frequency of our downloads.

There may be different results here for "busy" and "not so busy" clients.

Of course, the analysis needs to include the security impact.