Stats not being collected from all nodes, dangling nodes left in oper data store after mininet disconnects

Two intermittent issues are being observed with mininets with a large number of nodes:

1. Statistics are not being collected for some nodes. Everything else seems to be working fine, but a for a few nodes stats are not being collected.

2. When mininet exits, one or two nodes are left "dangling" in the oper space. Note that the inventory data (node connector, the node core data, etc.) are deleted, only statistics are left behind (most of the time just a partial set of table stats, e.g. 145 empty tables out of 254). Connecting/disconnecting mininet of the same size clears the condition. (i.e. where the dangling 'shadow' node is re-discovered, the old dangling stats are overwritten by a newly collected set of stats).