Proper serialization for evidence::Params and Duration

[greg-szabo]

evidence::Params is currently JSON serializes the domain struct. It could easily be converted to the tendermint_proto::types::EvidenceParams struct instead, since it's already its proto-type.

It contains a Duration which is also a mess: there's evidence::Duration which contains a std::time::Duration but all of them should fall back to the protobuf struct google::protobuf::Duration. This can be cleared up.

This is low priority, so we shouldn't do it for 0.17.0 but it would be nice to have afterwards.