Keep track of the tables we discover

NodeStatisticsHandler has always the most uptodate information about
which tables were discovered. Let's cache the table IDs -- we will
need them for sending out follow-up requests.

And now that we do that, we do not have to read the tables from
DataProviderService just to get the keys -- use whatever we cached.

Change-Id: I9550ae3330215277d373ec0b2ea67945ef0e7c5c
Signed-off-by: Robert Varga <rovarga@cisco.com>