- dns query log: robust file flushing mechanism

Before this patch we could exit the process without waiting for
 file writing task to complete.
As a result a file could become corrupted or a large chunk of data
 could be missing.

Now the main thread either waits until file writing task completes
 or it writes log buffer to file itself.

dnsforward/dnsforward.go

@@ -236,7 +236,7 @@ func (s *Server) stopInternal() error {
 	}
 
 	// flush remainder to file
-	return s.queryLog.flushLogBuffer()
+	return s.queryLog.flushLogBuffer(true)
 }
 
 // IsRunning returns true if the DNS server is running