Refactor stats collectors to use composition pattern (facebookincubator#441)

Summary:
Pull Request resolved: facebookincubator#441

Previously we modeled the StatsCollector classes after DWRF's implementation with a builder pattern that handles serialization directly.

Since Nimble has a very different use case and handles stats serialization through another adapter class, the current pattern just requires a composition of StatsCollection and their owning stats objects, where stats are exposed with read-only api. This makes extension/decorator classes like SharedStatisticsCollector and DeduplicatedStatisticsCollector cleaner and less error prune.

Reviewed By: macvincent

Differential Revision: D91536664

fbshipit-source-id: 620845ceb8e68ce505aeedff6824c7efb2aed62f

Author: Huameng (Michael) Jiang