Skip compressing/writing artifacts that are already in the cache.

Compressing things is expensive, so no reason clients should do it if they don't have to and writing the same thing over and over wastes IO capacity on the cache server.

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/39729173 passed

Bugs closed: 760

Reviewed at https://rbcommons.com/s/twitter/r/1272/