SOLR-6137: Schemaless concurrency improvements:
- Fixed an NPE when reloading a managed schema with no dynamic copy fields
- Moved parsing and schema fields addition to after the distributed phase
- AddSchemaFieldsUpdateProcessor now uses a fixed schema rather than always retrieving the latest, and holds the schema update lock through the entire schema swap-out process
(merged trunk r1610725 and r1610726)

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1610743 13f79535-47bb-0310-9956-ffa450edef68