Cache files to improve genieacs-fs performance

This also fixes a memory leak issue when the client disconnects before
completing the download. I suspect the leak had something to do with
using pipe() but I'm not sure if it's a bug in MongoDB driver or that
we were lacking proper error handling. Anyway, the pipe is no longer
leaking :)

bin/genieacs-fs.ts

@@ -94,7 +94,7 @@ if (!cluster.worker) {
 
   const _listener = (req, res): void => {
     if (stopping) res.setHeader("Connection", "close");
-    listener(req, res);
+    void listener(req, res);
   };
 
   const initPromise = db