prop224: Implement consensus params to adjust portion of the protocol

After discussion with developers and tor-dev@ thread here https://lists.torproject.org/pipermail/tor-dev/2016-December/011725.html, implementing consensus parameters to control some portion of the protocol. What comes to mind:

• Default number of introduction points and maximum number
• Timeframe and number of introduce before rotating an intro point.
• Descriptor lifetime.
• How many HSDir for a descriptor
• Rendezvous timeout time.

There are probably more.