[enh] API: provide an accessor to BlockchainService flow control

app/service/AbstractService.js

@@ -9,6 +9,11 @@ const fifo = async.queue(function (task, callback) {
 
 module.exports = function AbstractService () {
 
+  /**
+   * Gets the queue object for advanced flow control.
+   */
+  this.getFIFO = () => fifo;
+
   /**
    * Adds a promise to a FIFO stack of promises, so the given promise will be executed against a shared FIFO stack.
    * @param p