SOLR-6776: only clear changesSinceCommit on a hard commit so tlog will still be flushed on a softCommit

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene_solr_4_10@1642951 13f79535-47bb-0310-9956-ffa450edef68
SOLR-6653: should return error code >0 when something fails

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene_solr_4_10@1642967 13f79535-47bb-0310-9956-ffa450edef68