[medusa] tweaks to asyn{core,chat}.py

[Sam Rushing]

I've been trying to tighten up the async modules a bit to fight an
effect that shows up with lots of concurrent connections (>500).

I've made some notes:

http://www.nightmare.com/medusa/async_tweaks.html

I think the changes are a win, so I'll be checking in this code over
the next day or so. If anyone out there is violating the
asyncore.socket_map abstraction (relying on the way the dictionary is
used) or sees a problem I've missed, please let me know.

I think the only code in medusa that cares is the stuff in http_server
that kills zombie connections. Zope may have done something similar
with the ftp server?

-Sam