Better printing of AbstractBound in tracing

We currently trace messages like:

Tracing.trace("Executing seq scan across {} sstables for {}..{}", new Object[]{view.sstables.size(), startWith, stopAt});

This lack a little bit of precision in the sense that it doesn't say if the start/end are inclusive or exclusive (all of which we do support). Also, for order preserving partitioner, we allow startWith and stopAt to be actual keys, and in that case the output will be 'DecoratedKey(<token>, <key_in_hex))' which unnecessarily obscure. Attaching patch to switch to a nicer representation.