SolrCore.execute should wrap log message construction in "if (log.isInfoEnabled())"

As mentioned by Fuad on solr-user, there is some non-trivial log message construction happening in SolreCore.execute that should be wrapped in "if (log.isInfoEnabled())" ...

http://old.nabble.com/SOLR-Performance-Tuning%3A-Disable-INFO-Logging.-to26866730.html#a26866943

...the warn level message in that same method could probably also be wrapped since it does some large string building as well.