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 think it was a mistake to proliferate all those `LockType` and `ScopedLock` types. This was done pre-C++11. Now that we have <mutex> we should just use it.