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 
Either you should make this auto to correspond to the other auto variables, or you should make the other variables explicitly declared.

Since it's arithmetic, there's a good argument for explicit (non-auto) declarations.