Send Lucene's IndexWriter infoStream messages to Logger lucene.iw, level=TRACE

Lucene's IndexWriter logs many low-level details to its infoStream
which can be helpful for diagnosing; with this change, if you enable
TRACE logging for the "lucene.iw" logger name then IndexWriter's
infoStream output will be captured and can later be scrutinized
e.g. using https://code.google.com/a/apache-extras.org/p/luceneutil/source/browse/src/python/iwLogToGraphs.py
to generate graphs like http://people.apache.org/~mikemccand/lucenebench/iw.html

Closes #5891