configurable concurrency in the gateway-ipns plugin

plugins/gateway-ipns.js

@@ -32,7 +32,12 @@
             'https://ipfs.sloppyta.co/ipns/',       // UK
             'https://gateway.temporal.cloud/ipns/', // Germany
             'https://ipfs.best-practice.se/ipns/'   // Sweden
-        ]
+        ],
+        // how many simultaneous connections to gateways we want
+        // more concurrency means higher chance of a request succeeding
+        // but uses morebandwidth and other resources;
+        // 3 seems a reasonable default
+        concurrency: 3
     }
 
     // merge the defaults with settings from SamizdatConfig
@@ -55,15 +60,16 @@
         // https://<gateway_address>/<pubkey>/<rest_of_URL>
         var ipnsUrl = config.ipnsPubkey + url.replace(/https?:\/\/[^/]+/, '')
         
-        // pick 3 gateways, at random
+        // we don't want to modify the original gateways array
         var sourceGateways = [...config.ipfsGateways]
-        // if we have fewer than 3 gateways configured, use all of them
-        if (sourceGateways.length <= 3) {
+        
+        // if we have fewer than the configured concurrency, use all of them
+        if (sourceGateways.length <= config.concurrency) {
             var useGateways = sourceGateways
-        // otherwise get 3 at random
+        // otherwise get `config.concurrency` gateways at random
         } else {
             var useGateways = new Array()
-            while (useGateways.length < 3) {
+            while (useGateways.length < config.concurrency) {
                 // put in the address while we're at it
                 useGateways.push(
                     sourceGateways