[cobalt-client]: BaseHistogram and Counter for zircon.

Define a atomic Counter and BaseHistogram for zircon. All allocation
costs happen upfront when constructing the objects.

The Collector(TBI) will own the BaseHistogram and expose a Histogram
to the users that allows them increment and query the value of a given
bucket, etc.

TEST=cobalt_client_test

Change-Id: Iffcddca1b3f284776a655e201e1e5219c1246317