Assets are lost on history navigation

[rashidkpc]

There's a bug with asset writing that can cause the images assets from one workpad to clobber another

To reproduce:

1. Create a workpad with an image element
2. Create a new workpad with a different image
3. Open the first workpad.
4. Hit the back button in the browser
5. Observe warning triangle. Open asset manage, observe that first workpad now has the image from the 2nd.

It appears to be the entire asset key that is getting clobbered, so you could also make a new workpad with no images, do the back button thing, and observe the empty asset list. I just wanted to make it clear that what was happening was that the asset key from one workpad was clobbering others.

[elasticmachine]

Pinging @elastic/kibana-canvas