(web server) punts keep-alive to impls; http server uses (ice-9 poll)

(web server) punts keep-alive to impls; http server uses (ice-9 poll)

* module/web/server.scm: Rewrite to remove the extra "keep-alive"
  parameter. Instead, since the server is an essentially stateful
  object, have clients that want to do keep-alive manage that set as
  part of the server state. Also avoids imposing a particular data
  structure on the server implementation.

* module/web/server/http.scm: Adapt to the new server interface. Also,
  use a poll set instead of select and lists. Makes handling 1000
  clients at a time much more possible.