Start with making setup:di:compile extensible

[IvanChepurnyi]

Thinking about making setup:di:compile command extensible with default implementation being used internally.

This will allow implementing additional separate packages for each kind of compilation process, like:

• Compile configuration, generate merged config structures to use Redis only for database affected changes in config.
• Compile layout structures to make each handle as include and combine handles from includes instead of combinational explosion unique layout handles for each product and category.

And many more ways to improve stuff that is cached in Redis while can be generated during di compilation.