Provide "num_workers" configuration option

[ghost]

re #10085

num_workers is currently hard-coded to 4
https://github.com/paritytech/parity-ethereum/blob/7c335e87642b848f61ab3ee7bbff5964b53d6f11/util/io/src/service_mio.rs#L198_

A configuration-option should be provided, with a default of 4.

Implementation

Look at the num_verifiers config option as a guideline.

Compatibility

Using a default of 4, the behavior should be the same

[ghost]

@joshua-mir , this one is related to #10085 (which is still unsolved, possibly closed by automatism).

Edit: priority should be increased.

[debris]

eventually we want to get rid of util/io so this feature is not a priority