- set master_conn to None to clarify disconnection
- purge node pool after closing all connections
- allow restarting polling thread after its shutdown
Also, only start polling thread when needed (side-effect of last point).

git-svn-id: https://svn.erp5.org/repos/neo/trunk@2414 71dcc9de-d417-0410-9af5-da40c76e7ee4