Cleanup of luigi.mock

This change makes sure the manager (needed by MockFile) isn't instantiated in the global scope. Also a bunch of other cleanup to avoid pulling out internal variables in tests etc

luigi.mock would instantiate multiprocessing.Manager() in the global scope which ends up calling bind(). There is a limitation in the Linux kernel where named Unix sockets can't have longer names than 120 bytes, or else you will segfault. In Hadoop jobs, it turns out that the TMPDIR is actually a path longer than 120 bytes, which meant that any Hadoop job that accidentally imports luigi.mock will fail with a really obscure error.