We launch dummy descriptor fetches more often than needed

Right now, we have code in update_router_descriptor_downloads() to launch a fetch for authority.z if we want to learn our IP from a directory fetch. We do this if:

• We're a server
• We don't have the Address option set
• At least 20 minutes have passed since we last launched a router descriptor download
• At least 20 minutes have passed since we last launched a

Per discussion in bug legacy/trac#652 (moved), we could be even more quiet about launching these fetches. We could also require that

• At least 20 minutes have passed since we last launched any appropriate directory op.
• At least 20 minutes have passed since we got a new incoming connection on what we think our IP is.
• At least 20 minutes have passed since we got confirmation of our current IP in a NETINFO cell

We could also make the "20 minutes" value configurable by a networkstatus parameter.

This is a minor issue, since the current behavior is inelegant, but not actually hurting anything.