SOLR-6145: Fix Schema API optimistic concurrency by moving it out of ManagedIndexSchema.add(Copy)Fields() into the consumers of those methods: CopyFieldCollectionResource, FieldCollectionResource, FieldResource, and AddSchemaFieldsUpdateProcessorFactory.

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1601770 13f79535-47bb-0310-9956-ffa450edef68