Add Socket tagging for granular data accounting.

Introduces public API to apply "tags" to track data traffic originating
from the current thread.  (Under the hood, the tags are maintained and
applied in BlockGuard.)  Also adds tag/untag methods for developers who
maintain their own Socket pools.

Change-Id: Ic2dd3155559a93a7b613c7853748d4c44fb3a39e