Add log rotation to pgctl

[0xngold]

Changes:

• Switches from the internal pgctl-timestamp logger to s6-log (#188)
• Deletes pgctl-timestamp & associated code inside the pgctl core
• Fixes several new pylint warnings (methods named default like the one in the custom json encoder are still busted)
• Fixes tests that break due to XDG_RUNTIME_DIR being set (depending on import order, some code paths would see the XDG_RUNTIME_DIR from the environment, and others would read the blanked/unset version)

Questions:

• Due to the removal of pgctl-timestamp, this seems like a major version bump. Thoughts?

[0xngold]

@bukzor this should be ready for another look. I've bumped the allowed complexity for __locked_state_change, but I'm open to thoughts about how to refactor things to avoid needing to do that.

[0xngold]

This fixes #188.