Track fees being paid to get confirmed within N blocks.

Merry Christmas! :) 

What do you think of this? I find the output easier to understand and reason about. Will leave it running for a while. It converged on something slightly above min fee for the 10th percentile pretty fast. However lots of people are paying above the min fee currently so the median/50th percentile can end up quite high, especially if you start your node during a time when no block was found for a while so there's a backlog.

For me right now the 10th percentile to confirm within 3 blocks is the min fee. Others are higher :( What this tells me is we need higher blocks .... but at least with a "%ile of N blocks" data source we can think about making a (crappy) UI for letting users pick how fast they'd like a confirm. Hopefully any such UI would be a temporary thing to get us through the scaling issues over the next year, until eventually all "normal" spends confirm as fast as possible.

I'll work on a  few unit tests too.