Solr should have the option of logging all jars loaded

Solr used to explicitly log the filename of every jar it loaded. It seems that the effort to reduce the verbosity of the logs has changed this, now it just logs the count of jars loaded and the paths where they were loaded from. Here's a log line where Solr is reading from ${solr.solr.home}/lib:

2018-02-01 17:43:20.043 INFO  (main) [   ] o.a.s.c.SolrResourceLoader [null] Added 8 libs to classloader, from paths: [/index/solr6/data/lib]

When trying to help somebody with classloader issues, it's more difficult to help when the list of jars loaded isn't in the log.

I would like the more verbose logging to be enabled by default, but I understand that many people would not want that, so I propose this:

• Enable verbose logging for ${solr.solr.home}/lib by default.
• Disable verbose logging for each core by default. Allow solrconfig.xml to enable it.
• Optionally allow solr.xml to configure verbose logging at the global level.
  • This setting would affect both global and per-core jar loading. Each solrconfig.xml could override.

Rationale: The contents of ${solr.solr.home}/lib are loaded precisely once, and this location doesn't even exist unless a user creates it. An out-of-the-box config would not have verbose logs from jar loading.

The solr home lib location is my preferred way of loading custom jars, because they get loaded only once, no matter how many cores you have. Jars added to this location would add lines to the log, but it would not be logged for every core.