#5 Server should periodically update the contents of sites

[Julian Hyde]

Server should periodically update the contents of sites,
so that it would notice if new packages appear.

If the site is down (or is crippled -- for example, it
seems to have no packages) the site should be marked
disabled, and the server should check back in a while.

This will solve a current problem: for a few of the sites I
have registered, a bug prevented any packages being
associated with the site. This would be fixed the next
time the server crawled over the site.