Implement support for download RPC

This uses special parameters (Downloads.) to trigger file download
making it poossible to use from scripts declaratively.

Resolves: #171

lib/instance-set.js

@@ -123,6 +123,14 @@ class InstanceSet {
   values() {
     return this.set.values();
   }
+
+  clear() {
+    this.set.clear();
+  }
+
+  get size() {
+    return this.set.size;
+  }
 }
 
 module.exports = InstanceSet;