* Remove annoying compiler warnings
* Fix compiler warning (that will become an error)
Fixes https://github.com/paritytech/parity-ethereum/issues/10648
I'm not sure this fix is Good™ but rustc seems happy enough.
There's a deeper issue which may or may not be related to this: the Engine is not shutdown properly and the `StepService` thread keeps running indefinitely after Ctrl-C (so `update_sealing()` is called repeatedly for 300sec). I don't think this is related to Clique as I've seen this happen on mainnet as well, but I wonder if the effects of it are worse for a PoA network where the node can create new blocks all on its own?
* Use saturating_sub
* WIP
* Fix warning, second attempt
The idea here is to avoid using `Arc::get_mut()` (which does not work: fails every time here) and instead trust `drop()` to do the right thing.
This is a conservative change. I think this can be reformed further, e.g. by `impl Drop for StepService` and halt the thread there, or even skip `join()`ing the thread entirely and trust the `AtomicBool` to signal shutdown. I also have doubts abut the `Option<StepService>`: seems a bit much to have an `Option` there and it makes things cumbersome.
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