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 
Why did you make this change?  You just defined LockType, so use it!

Is it the case that the lock really has to be mutable?  If so, shouldn't *all* locks be mutabe?

You seem to be able to take a lock in a const method without this, which makes sense when you think about it...