Some requests can be safely ignored when received over a closed connection.
This was previously done explicitly in handlers, but it turns out it would
cause a lot of code duplication. Instead, define the policy on a packet
type basis, and apply it to all packets upon reception, before passing it
to handler.
Also, protect request handlers when they respond, as connection might be
closed.

git-svn-id: https://svn.erp5.org/repos/neo/trunk@2419 71dcc9de-d417-0410-9af5-da40c76e7ee4