Organize project in areas

[edongashi]

As the project is growing we get more and more files in a flat structure. I think we need to organize headers and source files logically.

Loose example of what I'm suggesting:

.
├── aggregation
│   ├── aid.h
│   ├── aid_tracker.h
│   ├── contribution_tracker.h
│   └── random.h
├── config.h
├── hooks.h
├── query
│   ├── context.h
│   ├── node_helpers.h
│   ├── oid_cache.h
│   ├── rewrite.h
│   └── validation.h
└── utils.h

Not happy with aggregation but that was the first thing which came to mind.

Thoughts?