support for message expressions

for #943

replaces https://github.com/Particular/NServiceBus/pull/1959

now this is supported 

    protected override void ConfigureHowToFindSaga(SagaPropertyMapper<TestSagaData> mapper)
    {
        mapper.ConfigureMapping<OtherMessage>(m => m.Part1 + "_"+m.Part2)
            .ToSaga(s => s.Key);
    }