Fix - Increment number in Slate keygenerator function

[v-stickykeys]

Problem

The keyInt wasn't being incremented and produced this error:

A node with a duplicate key of "1571842451212_0" was found! Duplicate keys are not allowed, you should use `node.regenerateKey` before inserting if you are reusing an existing node.

Solution

Define keyInt inside of the render method because the keygen function does not have access to the local state updates from MyApp.

Notes

See example here: https://github.com/zeit/next.js/blob/canary/examples/with-slate/lib/useCustomKeygen.js

[v-stickykeys]

@CraigLu Want to make sure this still meets what you need for the diffing.