Change sFlow model to reflect per-bridge sampling

Change sFlow model to reflect per-bridge sampling

Until now, we were presenting a separate sFlow data-source (sampler) for
each ifIndex-interface.  This caused problems with samples that did not
easily map to an ifIndex being aliased together and breaking the sFlow
containment rules.  This patch changes the model to present a single sFlow
data-source for each bridge.  Now we can still make all reasonable effort
to map packet samples to ingress/egress ifIndex numbers, knowing that the
fallback to "unknown" does not break the sFlow model.  Note that
interface-counter-polling is still handled the same way as before, with
sFlow counter-polling data only being exported for ifIndex-interfaces.

Signed-off-by: Neil Mckee <neil.mckee@inmon.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>