SOLR-6747: Add an optional caching option as a workaround for SOLR-6586.

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_5x@1640587 13f79535-47bb-0310-9956-ffa450edef68
SOLR-6365 implicit requesthandler (specified in code) takes lower precedence over <initParams>

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_5x@1640595 13f79535-47bb-0310-9956-ffa450edef68