Infinite delete queries

[olinux]

Depending on the execution time of a thread, in the current implementation a org.openrdf.rio.RDFHandlerException: java.io.IOException: Pipe not connected exception can happen (if the future task tries to flush before the pipeinputstream has been registered. Since the stream is not closed when an exception raises during query execution, the pipeinputstream will never be closed and therefore will stay open infinitely. This can result in a crash of blazegraph in the long run.