[REQUIREMENT] [Ruby] Revisit design and class diagrams, then refactor code

[jessiejuachon]

Is your feature request related to a problem? Please describe.

• [x] It is unclear what design patterns are used, if any, in the library.
• [x] Some terminologies are inconsistent (e.g. "source" vs "default" vs "fallback")
• [x] Some code logic implementations are duplicated / complex.
• [x] Mark internal methods as private
• [x] Add source comparison
• [x] Only one same request(component-locale combination) is allowed at the same time
• [x] Config class
  • It has many unnecessary code and needs to refractor.
  • Add necessary configuration items and remove uncessary configuration items.
• [x] Refractor cache module to use lock to make sure concurrency safe.
• [x] Refractor logger
• [x] Define new interfaces to make client easy-to-use.

Describe the solution you'd like A clear and concise product design that may include class diagrams and and/or design patterns used. Multiple small refactor-related PRs.