Disconnect node statistics tracking

This patch moves scheduling into the node handlers themselves. This
should allow for spreading out of the statistics collection, preventing
spikes. Additionally delay the initial collection by up to 5 seconds to
deal with multiple switches connecting at the same time.

Change-Id: I6ea039b405595b3756263bddd9cf8997bc73a038
Signed-off-by: Robert Varga <rovarga@cisco.com>