This lets us make a lot of other stuff const, allows the compiler to
generate (slightly) better code, and will make me get slightly fewer
patches from folks who stick mutable stuff into or_options_t.
const: because not every input is an output!
To turn this on, set UseMicrodescriptors to "1" (or "auto" if you
want it on-if-you're-a-client). It should go auto-by-default once
0.2.3.1-alpha is released.
Because of our node logic, directory caches will never use
microdescriptors when they have the right routerinfo available.
Previously on 0.2.2, we'd never clean the cache. Now that we can
clean it, we want to add a condition to rebuild it: that should happen
whenever we have dropped enough microdescriptors that we could save a
lot of space.
No changes file, since 0.2.3 doesn't need one and 0.2.2 already has some
changes files for the backport of the microdesc_clean_cahce() function.
