Split routerlist.c and dirserv.c into smaller modules

These are a couple of the biggest modules in Tor, and they have a bunch of responsibilities each.

Per discussion, we're going to try to put more code movement into 0.3.5, to reduce the number of code-movement releases we need to merge across.

See branch split_routerlist_dirserv with PR at https://github.com/torproject/tor/pull/363 .