Major refactoring to ConfigSetService -- SOLR-15258

[David Smiley <ds...@apache.org>]

Just an FYI of a major change coming and opportunity for feedback.  I've
worked with Nazerke on SOLR-15258 to do a major refactoring of
ConfigSetService to have C.R.U.D. operations for ConfigSet management.  The
point is to make it more viable to plug in a custom ConfigSetService
(recently made possible via SOLR-15100).  An example of a realistic
implementation would be a file system + ZK hybrid that augments the visible
configSets with those from the file system (read-only there).  This
issue/PR does not provide that but it could be in a future one if others
like it.

I plan to merge Friday, subject to review of course.  The changes touch
some 53 files but to many of those, it's very small.  Some of the bigger
changes aside from ConfigSetService itself, of course, were in
ConfigSetsHandler, and BackupManager.  The changes are purely internal.

 https://issues.apache.org/jira/browse/SOLR-15258

~ David Smiley
Apache Lucene/Solr Search Developer
http://www.linkedin.com/in/davidwsmiley