Utility to log deprecation warnings

Added org.apache.solr.logging.DeprecationLog. Up to the user to configure logging to go to a separate file if they wish.
Old description:

As discussed in solr-user list:

When instructing people in what to do before upgrading to a new version, we often tell them to check for deprecation log messages and fix those before upgrading. Normally you'll see the most important logs as WARN level in the Admin UI log tab just after startup and first use. But I'm wondering if it also makes sense to introduce a separate DeprecationLogger.log(foo) that is configured in log4j2.xml to log to a separate logs/deprecation.log to make it easier to check this from the command line. If the file is non-empty you have work to do