Don't use the stored uri from parsoid config

 * The parsoid config is used across requests and storing anything there
   is setting yourself up for race conditions.

 * This was causing major corruption on wiki.

 * The refactor in 915ea3f went from
   using a bound copy of the uri to that stored on the parsoid config,
   causing some concurrent requests to use the wrong API.

Change-Id: Ia2244b9737fee1563ca3a33ba3b8683862b39d24

lib/mediawiki.parser.environment.js

@@ -278,11 +278,11 @@ MWParserEnvironment.prototype.switchToConfig = function(prefix, cb) {
 	var nothingToDo = {};  // unique marker value
 	var parsoid = env.conf.parsoid;
 
-	var getConfigPromise;
+	var getConfigPromise, uri;
 	if (!prefix) {
 		getConfigPromise = Promise.reject(new Error('Wiki prefix not provided'));
 	} else {
-		var uri = parsoid.interwikiMap.get(prefix);
+		uri = parsoid.interwikiMap.get(prefix);
 		if (!uri) {
 			// SSS: Ugh! Looks like parser tests use a prefix
 			// that is not part of the interwikiMap -- so we
@@ -318,7 +318,7 @@ MWParserEnvironment.prototype.switchToConfig = function(prefix, cb) {
 
 	return getConfigPromise.then(function(resultConf) {
 		if (resultConf === nothingToDo) { return; }
-		env.conf.wiki = new WikiConfig(env, resultConf, prefix, parsoid.apiURI,
+		env.conf.wiki = new WikiConfig(env, resultConf, prefix, uri,
 				env.getAPIProxy(prefix));
 		env.confCache[prefix] = env.conf.wiki;
 	}).nodify(cb);