Row overlapping when dynamic row height or virtualization.

**I’m submitting a [x] bug report => search github for a similar issue or PR before submitting [ ] feature request [ ] support request => Please do not submit support request here, post on Stackoverflow or Gitter

**Current behavior**
Sometimes two row are overlapping each other while scrolling or hovering. This is happening when the `rowHeight ` is dynamic or `virtualization ` flag is `true`. I am attaching screenshots of the problem.

**Expected behavior**
Row should not overlap each other.

**Reproduction of the problem**
Visit [virtual-paging](http://swimlane.github.io/ngx-datatable/#virtual-paging) or [Dynamic Row Height](http://swimlane.github.io/ngx-datatable/#dynamic) scroll down to the end of the table then scroll up and down until it shows the mentioned problem. 

**What is the motivation / use case for changing the behavior?**
<!-- Describe the motivation or the concrete use case -->

**Please tell us about your environment:**
<!-- Operating system, IDE, package manager, HTTP server, ... -->
Windows 10
Node: 8.11.2

* **Table version:** 
<!-- Check whether this is still an issue in the most recent table version -->
14.0.0

* **Angular version:** 
<!-- Check whether this is still an issue in the most recent Angular version -->
Angular CLI: 6.1.4
Angular: 7.0.1

* **Browser:** [all | Chrome XX | Firefox XX | IE XX | Safari XX | Mobile Chrome XX | Android X.X Web Browser | iOS XX Safari | iOS XX UIWebView | iOS XX WKWebView ]
Chrome
<!-- All browsers where this could be reproduced -->

* **Language:** [all | TypeScript X.X | ES6/7 | ES5]
TypeScript

![screenshot 37](https://user-images.githubusercontent.com/18557453/48415073-6618f600-e772-11e8-913f-d6c37ad47536.png)
![screenshot 38](https://user-images.githubusercontent.com/18557453/48415074-6618f600-e772-11e8-80a0-71de63940655.png)

[mfjharvey]

I’ve posted a potential workaround for this here.

[pwinstead]

Looks to be the same issue described in this other issue.

Duplicate of #1594