Fix concurrent compiles. Also configure 2 by default

etc/config/gcc-explorer.defaults.properties

@@ -9,3 +9,4 @@ cacheMb=200
 #androidNdk=/opt/google/android-ndk-r9c
 language=C++
 options=-O2
+maxConcurrentCompiles=2

lib/compile.js

@@ -88,7 +88,7 @@ Compile.prototype.runCompiler = function (compiler, options) {
         stderr += data;
     });
     return new Promise(function (resolve, reject) {
-        child.on('error', function(e) {
+        child.on('error', function (e) {
             reject(e);
         });
         child.on('exit', function (code) {
@@ -120,7 +120,7 @@ Compile.prototype.compile = function (source, compiler, options, filters) {
     }
     self.cacheMisses++;
 
-    var tempFileAndDirPromise = self.compileQueue.add(function () {
+    var tempFileAndDirPromise = Promise.resolve().then(function () {
         return self.newTempDir().then(function (dirPath) {
             var inputFilename = path.join(dirPath, props.get("gcc-explorer", "compileFilename"));
             return self.writeFile(inputFilename, source).then(function () {
@@ -176,16 +176,18 @@ Compile.prototype.compile = function (source, compiler, options, filters) {
         });
     });
 
-    return compileToAsmPromise.then(function (result) {
-        if (result.dirPath) {
-            fs.remove(result.dirPath);
-            result.dirPath = undefined;
-        }
-        if (result.okToCache) {
-            self.cache.set(key, result);
-            self.cacheStats();
-        }
-        return result;
+    return self.compileQueue.add(function () {
+        return compileToAsmPromise.then(function (result) {
+            if (result.dirPath) {
+                fs.remove(result.dirPath);
+                result.dirPath = undefined;
+            }
+            if (result.okToCache) {
+                self.cache.set(key, result);
+                self.cacheStats();
+            }
+            return result;
+        });
     });
 };