V2 - reduce self repeater traffic

[mikecarper]

Repeaters Adverts and telemetry seem to be the biggest problem in very large mesh networks. Here's some ideas for fixing this:

• 64bit encryption for repeaters to shrink advert packets vs 256bit encryption
• making telemetry on repeaters not encrypted by default
• take into account channel usage and network size for automatic back off of advert frequency
• use mqtt nodes for automatic telemetry of repeaters; say dm this node once every 18/36/72 hours
• use diff of telemetry so a graph can be created from one packet

nice to have would be the ability to have repeaters pick their prefix independently from the key. Would reduce complexity of changing prefixes. Will need to have some sort of scheme that essentially adds the bytes to the beginning of the key so if things change in the future this won't break