stats: added IStatsProducer interface, fixed stats provider startup

stats: added IStatsProducer interface, fixed stats provider startup

this adds an interface, IStatsProducer, defining the get_stats() method
which the stats provider calls upon and registered producer, and made the
register_producer() method check that interface is implemented.

also refine the startup logic, so that the stats provider doesn't try and
connect out to the stats gatherer until after the node declares the tub
'ready'.  this is to address an issue whereby providers would attach to
the gatherer without providing a valid furl, and hence the gatherer would
be unable to determine the tubid of the connected client, leading to lost
samples.