don't pass internal state object to barracks when calling start()

[yoshuawuyts]

Expected behavior

Rehydration should be possible without passing an empty state object to barracks - means we're running an extra for loop on startup and not a fan of the extra overhead - signals something is broken and we used ducktape to fix it.

Actual behavior

This line is required - https://github.com/yoshuawuyts/choo/blob/master/index.js#L65

Steps to reproduce behavior

Remove the line, run the tests

[yoshuawuyts]

Closing because choo@5 will introduce a new API. See #425 for the merged PR. Thanks!