Remove TransformerLogger's ability to change level?

[oxinabox]

If we took this ability away from TransformerLogger, then we would be able to have its min_enabled_level set to something more simple. To disable it we could check the level after the transform and error if it changes.

We could introduce it back in a PromotingLogger (aliased to DemotingLogger?) which you would use one of the filtered to pipe things into and then those would get promoted/demoted and then you could feed them back into the main logger?