Implement HTrace based tracing

Small issue in that not everyone serializes htrace annotations, but that's an
open question of the right way to do that anyways

Adding tracing to:
 - MutationState
 - query plan tracing
 - iterators

Metrics writing is generalized to support eventual hadoop1 implementation.
Also, supporting test-skipping with a custom Hadoop1 test runner + annotation

Default builds to hadoop2, rather than hadoop1 (particularly as hadoop1 is
now a second-class citizen).
Looks like writer can be final. Also, is this class meant to be thread safe? If yes, maybe annotate with @ThreadSafe?