SOLR-6631: Part deux - refactor LatchChildWatcher to LatchWatcher that takes an optional EventType during construction to only release the latch when a specific event type is received.

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_5x@1635576 13f79535-47bb-0310-9956-ffa450edef68
SOLR-6584 Export handler causes bug in prefetch with very small indexes.

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_5x@1635587 13f79535-47bb-0310-9956-ffa450edef68