Mitosheet default dataframe renderer gets wrong df name

[aarondr77]

The default dataframe output Mito spreadsheet uses the the execution number to find the dataframe variable name. The assumption was that the execution number was unique in the notebook. But that is not true!

Steps to recreate.

1. Run some code cells
2. Restart the kernel and refresh the browser
3. Notice that the execution #s from the previous runs are still in the notebook
4. Run a code cell. Notice that the execution number starts back at 1.

As a result, if you create a Mito spreadsheet default renderer, it will find the last line of the first code cell with matching execution number.

To fix this, we should move away from using the execution number.