Make CacheKey hash and pickle friendly

Don't need payloads on cache_keys, so removing them makes them pickle-friendly.

Testing Done:
tests/python/pants_test/cache::

Passed: https://travis-ci.org/pantsbuild/pants/builds/38538962

Reviewed at https://rbcommons.com/s/twitter/r/986/
Sync of https://github.com/pantsbuild/pants/commit/bdf68f00a6bdeeef8c165ce6e7abe021297760f9