Factor our hw_port_stats_fb303 lib
Factor our hw_port_stats_fb303 lib

Summary:
Implementation based on what we do in bcm layer
for reporting (non histogram) port stat counters. Abstracting
this lib out so we can reuse this code in both SAI and bcm
layers. Plus pulling this out of BcmPort makes it much
easier to unit test this code.

Reviewed By: boryas

Differential Revision: D19676641

fbshipit-source-id: 88aa20f20339729f5387500bcf6084871f4e7caa

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