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 
I don't really know what the type of `v.second` is. And the great thing is that I was able to work on this code without knowing. Which is moderately strong evidence that the lack of a type was no impediment.