For the moment, this only fixes a performance issue because in the worst case,
subsequent pings (every 6 seconds) would make _poll() check in the connection
has any pending message (while processing 'rlist').

git-svn-id: https://svn.erp5.org/repos/neo/trunk@2760 71dcc9de-d417-0410-9af5-da40c76e7ee4