Core APIs migration

[Jubilee101]

Introduce value system, migrate JSON, deque and ART from pgmoneta.

[Jubilee101]

Deque, ART and json are imported. I didn't replace cJSON in management.c to avoid breaking stuff. But it can be removed once management is refactored. What were once in json.c/h are move to management.c as static functions since they are only used there. I'll keep this issue open, so any questions just let me know.

[Jubilee101]

Note that we found a small bug about indentation when printing in text format. It's fixed but I want to watch for a while in case something else comes up. If you are interested, check out pgmoneta#380. We try to make the text format yaml-like but it turns out to be a little harder than json.