Define statuses for RFCs

Created by: nicksnyder

I have received feedback from multiple people that our RFC process lacks clarity on when RFCs are "done" or "approved".

This PR proposes standardized on a small number of statuses and adds more formality to the approval process.

If approved, I will update the titles of existing RFCs to use these terms.

@slimsag @efritz @unknwon IIRC you all had questions for me about this recently. Would these proposed changes address your concerns?