Reorganize and clean up package

[shifucun]

Mixer has grown a lot in the past years. The current package layout does not reflect the code well:

• The Sparql translator codes is spread out and should be consolidated.
• The API server contains a flat folder "server" with all the codes for 20+ APIs, it's getting very hard to navigate and maintain.

Need to re-organize the folder by introducing more tiers of packages based on the functions.