Fetch running-routers.

Fetch running-routers.

Split logic to initiate dirfetch, running-routers fetch, and
descriptor post.  arma: There are some XXXs here that raise design
questions which we should solve before the next release.

The biggest problem is this: Right now, the directory is about 50X as
large as running-routers uncompressed, and about 36X as large
compressed.  Assuming:

    - everybody gets the compressed version of everything,

    - everybody gets cached directories from random dirservers and
      uncached r-r from authdirservers

    - everybody downloads r-r at the same rate they now download dirs,

then using r-r from will *increase* authdirserver directory bandwidth usage
if there are significantly more caches than authdirservers.

I think it's safe to leave this in for now, since there aren't 3x36 caching
dirservers, but we should make everybody with a dirport cache running-routers
soon.  But I could be wrong.

svn:r2872