Refactor FlexCounter class (#1020)

FlexCounter class is a representation of flex counter group which supports querying multiple types of statistic/attributes. It supports multiple statistic/attribute types such as port counter, port debug counter, queue counter, queue attribute and so on. For each statistic/attribute type, it defines several member functions:

setXXXCounterList: e.g. setPortCounterList, setPortDebugCounterList
removeXXX: e.g. removePort, removeQueue
collectXXXCounters: e.g. collectPortCounters, collectQueueCounters
collectXXXAttr: e.g. collectQueueAttrs, collectPriorityGroupAttrs
so on

For different statistic/attribute types, these functions have very similar logic. This design document describes a proposal about how to refactor FlexCounter class by moving duplicate/similar logic to a single place.