Optionally configure connection as path to Unix socket (tbd: actual connection)
Better placing + testability of socket as a cli parameter
Perform actual connection (+ increase test coverage)
Remove test cases for the Unix socket config setting.

This is mostly something you would want to test in the redis-py project,
not in RQ.  The test cases make it a bit complex to read them and they
falsely suggest something interesting might be going on in them.
Merge branch 'Plexical-config-socket'
Update CHANGELOG.