Difficulty history

[BeaudanBrown]

Bit of a chaotic PR
PoW difficulty is now loaded from the DNS record in the format {"timestamp":difficulty,"timestamp":difficulty}
This should represent at least the last 4 days of difficulties
When verifying a client message, we always use the most recent difficulty
When verifying messages pushed by other snodes we use the smallest of either: the most recent difficulty in the past OR any difficulty 15 minutes before or after the message timestamp
This is to allow for messages that were accepted by snodes that hadn't updated their difficulty yet
The difficulty timer is handled by the worker thread and then posts the result to the main thread to update the ServiceNode value

Don't know how I feel about passing in length 1 vector for the client verification, just wanted to get it working for now