Add proxies for statistics getters

This is a simple unification of stats collection triggers so they are
centered about their per-node collector.

Change-Id: Iaf63d782b58b84ed3270b9215587f96f93aab3cc
Signed-off-by: Robert Varga <rovarga@cisco.com>
Split out statistics tracking into separate classes

- FlowStats
- FlowTableStats
- GroupDescStats
- GroupStats
- MeterConfigStats
- MeterStats
- NodeConnectorStats
- QueueStats

are all now tracked by dedicated classes. Also create a dedicated
QueueStatsEntry and FlowStatsEntry class.

Furthermore the flow walking code is modified to stop searching as soon
as it finds a matching flow.

Change-Id: Ia2bacb12f9c0af6520c115c161388b72a846b062
Signed-off-by: Robert Varga <rovarga@cisco.com>