Move two generic task mixins out of backend/core.
Review Request #3176 - Created Nov. 24, 2015 and submitted
There is an external user of ReplTaskMixin, so we leave a forwarding alias and deprecation message for it. There is no external usage of MutexTaskMixin, fortunately.
CI passes: https://travis-ci.org/pantsbuild/pants/builds/93064609
Manually ran repls and their tests.