Use trusted validators median fee

This fixes the problem with people running validators that don't use the temporary new fee floor. Note that this has already been deployed:
https://github.com/ripple/rippled/commits/release

Reviewers: @JoelKatz @tdfischer
Approvers: @rec @nbougalis 
This is an incorrect calculation of the median!  By always rounding down when you divide by two, you're statistically going to underestimate the median over the long term.

(Points for the comment, without which I might not have tripped back to an old stats class.)

The median should be calculated this way:
    auto size = fees.size();
    auto half = size / 2;
    if (size % 2)
        return fees[half];  
        // Odd number of fees: return the middle item.
    else
        return (fees[half - 1] + fees[half]) / 2;  
        // Even number of fees: return the average of the two items around the middle.