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 means that validators could arrive at a value for the fee that no individual validator has chosen. Its important that the median calculation always produce a value that at least one validator has selected (I think...@joelkatz ?)