Allow queueing of event channels

Steps to reproduce:
1. Create a new Behavior graph and open it
2. From the blackboard, Create a new EventChannel (named = "DefaultEventChannel", message = "On Event raised") and an integer variable "Event Count"
3. Add a StartOnEvent node and assign the EventChannel, then add and attach a log message to it.
4. Create an EventChannel object (from project view, right-click > Create > Behavior > Event Channel)
5. Assign the EventChannel object to the graph variable.
6. Create a EventTester Monobehavior (script below)
7. Assign the EventTester script to an object on the scene and set it's EventChannel variable with the object. Also set EventCounToSend variable to 2.
8. Enter playmode and press Space.

Actual results: 

• Event log message is printed only once.
  Expected results: 
• Event log message is printed twice.

Reproducible with versions: 
1.0.9

EventTester.cs

using UnityEngine;

public class EventTester : MonoBehaviour
{
    [SerializeField]
    DefaultEventChannel m_Channel;
    [SerializeField]
    int m_EventCountToSend = 1;

    void Update()
    {
        if (Input.GetKeyDown(KeyCode.Space))
        {
            for (int i = 0; i < m_EventCountToSend; ++i)
            {
                m_Channel.SendEventMessage();
            }
        }
    }
}