- Previous implementation was not able to import transactions with many small
  objects, the client for faster to send a store request than to process its
  answer. If X is the difference of time for these 2 operations, the maximum
  number of objects a transaction could contain was CRITICAL_TIMEOUT / X.
  And HasLock feature can't act as a workaround because it is not working yet.
- Change API of 'on_timeout', which currently only used by HasLock.
- Stop pinging when we wait for an answer. This wastes resources and would
  never recover any bad state.
- Make client connections send pings when they are idle instead.
  This implements keep-alive feature for high availability.
  Start with an non-configurable period of 60 seconds.
- Move processing of ping/pong to handlers.

git-svn-id: https://svn.erp5.org/repos/neo/trunk@2762 71dcc9de-d417-0410-9af5-da40c76e7ee4