Use Mempoolspace fees (#5490)

* Use Mempoolspace fees

Since bitcoind's fee estiomates are horrible, I would use an altenrative, but that adds a third party to the mix. We can either:
* Accept the risk (it is only for fee estimation anyway)
* Offer a toggle in the server settings
* Move this code to a plugin

* refactor

* Refactor

---------

Co-authored-by: nicolas.dorier <nicolas.dorier@gmail.com>

BTCPayServer/Controllers/UIInvoiceController.UI.cs

@@ -1266,7 +1266,7 @@ namespace BTCPayServer.Controllers
                 {
                     metadataObj = JObject.Parse(model.Metadata);
                 }
-                catch (Exception e)
+                catch (Exception)
                 {
                     ModelState.AddModelError(nameof(model.Metadata), "Metadata was not valid JSON");
                 }