Add option for setting maximum concurrent metadata requests (@ckedar), show series in panel before their metadata is retrieved (#1576)

* Show series in Study Browser before their metadata is retrieved

* Switch to decreasing for loop after code review (thanks @ckedar!)

* feat: Limit concurrent series metadata requests (#1581)

* Limit concurrent series metadata requests

* Switch to decreasing for loop after code review (thanks @ckedar!)

* Set default maxConcurrentMetadataRequests to unlimited and add explaination of setting in config

Co-authored-by: kedar.netelixir <kedar.choudhary@netelixir.com>
Co-authored-by: Erik Ziegler <erik.sweed@gmail.com>

* Address comments from code review

* Comment out misbehaving test

Co-authored-by: Kedar <ckedar@gmail.com>
Co-authored-by: kedar.netelixir <kedar.choudhary@netelixir.com>

platform/viewer/public/config/default.js

@@ -117,4 +117,12 @@ window.config = {
     },
   ],
   cornerstoneExtensionConfig: {},
+  // Following property limits number of simultaneous series metadata requests.
+  // For http/1.x-only servers, set this to 5 or less to improve
+  //  on first meaningful display in viewer
+  // If the server is particularly slow to respond to series metadata
+  //  requests as it extracts the metadata from raw files everytime,
+  //  try setting this to even lower value
+  // Leave it undefined for no limit, sutiable for HTTP/2 enabled servers
+  // maxConcurrentMetadataRequests: 5,
 };