run upto 5 apptasks in parallel

fixes #482

src/locker.js

@@ -9,6 +9,7 @@ function Locker() {
     this._operation = null;
     this._timestamp = null;
     this._watcherId = -1;
+    this._lockDepth = 0; // recursive locks
 }
 util.inherits(Locker, EventEmitter);
 
@@ -24,6 +25,7 @@ Locker.prototype.lock = function (operation) {
     if (this._operation !== null) return new Error('Already locked for ' + this._operation);
 
     this._operation = operation;
+    ++this._lockDepth;
     this._timestamp = new Date();
     var that = this;
     this._watcherId = setInterval(function () { debug('Lock unreleased %s', that._operation); }, 1000 * 60 * 5);
@@ -35,17 +37,31 @@ Locker.prototype.lock = function (operation) {
     return null;
 };
 
+Locker.prototype.recursiveLock = function (operation) {
+    if (this._operation === operation) {
+        ++this._lockDepth;
+        debug('Re-acquired : %s Depth : %s', this._operation, this._lockDepth);
+        return null;
+    }
+
+    return this.lock(operation);
+};
+
 Locker.prototype.unlock = function (operation) {
     assert.strictEqual(typeof operation, 'string');
 
     if (this._operation !== operation) throw new Error('Mismatched unlock. Current lock is for ' + this._operation); // throw because this is a programming error
 
-    debug('Released : %s', this._operation);
+    if (--this._lockDepth === 0) {
+        debug('Released : %s', this._operation);
 
-    this._operation = null;
-    this._timestamp = null;
-    clearInterval(this._watcherId);
-    this._watcherId = -1;
+        this._operation = null;
+        this._timestamp = null;
+        clearInterval(this._watcherId);
+        this._watcherId = -1;
+    } else {
+        debug('Recursive lock released : %s. Depth : %s', this._operation, this._lockDepth);
+    }
 
     this.emit('unlocked', operation);