Make uploads work with very slow upload speeds

chunk uploads get a timeout of 2mins (derived from http.timeout).
On servers like kimsufi, uploads takes forever (100 MB/sec limit).
Currently, our upload code does not dynamically adapt itself to
changing the concurrency when network is slow.

src/backups.js

@@ -269,7 +269,7 @@ function sync(backupConfig, backupId, dataDir, callback) {
                 });
             }
         }, iteratorCallback);
-    }, 10 /* concurrency */, function (error) {
+    }, backupConfig.syncConcurrency || 10 /* concurrency */, function (error) {
         if (error) return callback(new BackupsError(BackupsError.EXTERNAL_ERROR, error.message));
 
         callback();