Add ShareCenter::isShared($ajxpNode) to test if a node is shared

Signed-off-by: Etienne CHAMPETIER <etienne.champetier@fiducial.net>
Add PURGE_AFTER_SOFT option

The idea here is to remove file if they're not shared and older than PURGE_AFTER_SOFT,
and remove file unconditionally if older than PURGE_AFTER

Signed-off-by: Etienne CHAMPETIER <etienne.champetier@fiducial.net>