Restore a trace class for Hive to work

Shame on me that I let HIVE-9082 slip through a couple of releases.

Backstory: Hive (Pig does this too) has some nice code lifted from HBase which accepts some Java class and figures out the jar which contains that class (which is on the classpath) and then adds that jar as a necessary dependency for the query execution (e.g. libjars for mapreduce).

The problem is that when I did the Hive stuff, I chose a bad class: trace/src/main/java/org/apache/accumulo/trace/instrument/Tracer.java. This went away in the HTrace switchover. Because Hive has had a few releases since when this went in, if we don't restore this class, users will have to jump through hoops to make 1.7.0 work with those intermediate versions of Hive (0.14.0, 1.2.0)

At this point, it's much easier to make an empty class in Accumulo that will allow Hive to continue operating as-is. After enough time/releases, we can remove it again from Accumulo.