Serialized messages are generated even though they are not logged

sergey.soldatov made a good find through some careful log reading.

In ProtobufTranslationImpl, some TRACE logging that I had added in CALCITE-1159 is causing a bunch of garbage to be created on the heap.

The trace logging messages should be wrapped in a conditionals to avoid the generation of the message itself. I was a little too overzealous thinking that the slf4j api was saving me.