* Use Drop to shutdown stepper thread
Make period == 0 an error and remove the Option from step_service
* Remove StepService
Remove StepService and spawn the stepping thread in `Clique::new()`. Don't store the thread handle and instead trust the `AtomicBool` to signal shutdown time.
Don't check for `period > 0`: we assume a valid chainspec file.
* Don't shutdown the stepper thread at all, just let it run until exit
Also: fix a few warnings and tests
* Put kvdb_memorydb back
* Warn&exit when engine is dropped
Don't sleep too long!
* Don't delay stepping thread
* Better formatting
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