switched to open sourced rate limit module

[sampocs]

Context

Switched over the the rate limit module that's moved into it's own separate repo.

Noteable changes to the version in the isolated repo is it removed the legacy gov handlers and epochs dependency.

Brief Changelog

• Wired different keeper into app.go
• Updated import statements
• Removed legacy gov proposals types and rate limit epoch hooks
  • The open sourced module uses the non-legacy gov tx format and removes the epoch dependency with a lighter weight implementation
• Removed x/ratelimit and proto/stride/ratelimit
• Added migration to new module in upgrade handler

[ethan-stride]

LGTM