Critical Description
When using the embedded Solr server it can happen that ungraceful shutdown of repository and/or index (e.g. when SolrServer#shutdown is not called) can result in leftover write.lock file in the dataDir and hence it's not possible to restart the same embedded Solr server (even upon repository restart) unless the lock file is manually removed.
EmbeddedSolrServerProvider should check for such leftover lock files and eventually remove them upon server start.