Per-connection timeout support (instead of per-packet).

- Rename IdleEvent to IdleTimeout from event.py to connection.py
- Move connection-related logic in Connection itself and keep only
time-related logic in IdleTimeout
- Clarify differences between hard and soft timeouts.
- Remove (unused) 'additional_timeout' from ask()
- Remove (now useless) event_dict attribute from Connection.
- Remove external ping support, as the answer can not be handled at
application level.
- Expectation after a new incoming connection moved from Handler to
Connection.
- Fix (and clean) related tests.

git-svn-id: https://svn.erp5.org/repos/neo/trunk@1895 71dcc9de-d417-0410-9af5-da40c76e7ee4