Make directory objects expire faster

In tor_dirmgr::storage we have:

pub(crate) const EXPIRATION_DEFAULTS: ExpirationConfig = {
    ExpirationConfig {
        // TODO: Choose a more realistic time.
        router_descs: Duration::days(3 * 30),
        // TODO: Choose a more realistic time.
        microdescs: Duration::days(3 * 30),
        authcerts: Duration::ZERO,
        consensuses: Duration::days(2),
    }
};

We should lower the router_descs and microdescs defaults: as it stands, the microdescs one retains too much information and make our directory caches huge.

The C Tor implementation uses 7 days for the microdesc expiration cutoff, and 5 days for the router descriptor cutoff. I have no reason to think those are especially good or principled cutoffs, though.