SOLR-5682: Make the admin InfoHandler more pluggable / derivable.

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1565067 13f79535-47bb-0310-9956-ffa450edef68
SOLR-5691: Sharing non thread safe WeakHashMap across thread can cause problems. 

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1565069 13f79535-47bb-0310-9956-ffa450edef68