SOLR-6752: Buffer Cache allocate/lost metrics should be exposed.

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1642689 13f79535-47bb-0310-9956-ffa450edef68
SOLR-6554: Refactored Overseer to improve reuse and put batching logic inside ZkStateWriter. Added a new ZkStateWriterTest.

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1642693 13f79535-47bb-0310-9956-ffa450edef68