Rewrite Worker's core functionalities in C++

[raulchen]

See this proposal doc for context, motivation, and design.

Roadmap:

• Add high-level code structure with empty implementations. Methods will be split and organized into multiple categories.
• Implement methods in each category, based on current mechanisms.
  • Task submission.
  • Task execution.
  • Object store
  • Others.
• Migrate Java worker on top of C++ core worker.
• Migrate Python worker (Python code is currently more messy, and needs a major refactor).
• Try and benchmark alternative mechanisms (for task submission, object store, etc) based on the new abstraction.