Solr indexer thread should not keep the same ExecutionContext forever

The Solr indexer thread is a daemon thread which is in charge of indexing all the document put in a queue. Right no it's initializing an Execution when starting and keep it forever, among other things it means that component can keep storing things in a never cleaned execution context (for example I just got a 40MB ExecutionContext instance in myxwiki.org) and isolation is less good.