Scans spend significant time constructing debug string.

While profiling a Fluo test running lots of little scans, I noticed a string builder operation showing up prominently in the profiling results.  Below is a link to the problematic code.  Calling range toString was the most expensive part followed by KeyExtent toString.

https://github.com/apache/accumulo/blob/rel/1.7.3/core/src/main/java/org/apache/accumulo/core/client/impl/ThriftScanner.java#L405

I am not sure if we can change this in 1.7 and 1.8/1.9 because people may rely on this for debugging.  In 2.0 we may want to consider removing this (or moving it inside the logging code block).

Also, while looking at this I noticed that some of the log statements called String.format.  Those should be placed in a if(llog.traceEnabled()) block.