Extract sidekiq-cluster to top-level directory

While working on #343759 (closed), we ran into LOAD_PATH issues where due to sidekiq-cluster redefining some dependencies in lib/, and code in lib/ being auto-loaded in sidekiq workers, we ended up with duplicate definitions of some classes such as Settings.

After some discussion, we decided that the cleanest approach would be to extract all sidekiq-cluster related code into its own top-level directory, because:

• It addresses the immediate problem of not putting sidekiq-cluster specific code on LOAD_PATH automatically
• With the ongoing work in &6409 (closed), it is becoming a more self-contained application with added complexity, so it makes sense to live in its own tree
• It was never really library code to begin with, and it was sitting in lib/ mostly for convenience