KTOR-8194 Fix SelectorManager dispatcher ignored on Native

[Thomas-Vos]

KTOR-8194 SelectorManager creates own thread instead of provided dispatcher on Native

Now Native now longer creates a separate thread per SelectorManager, but can use IO pool when IO dispatcher is given, just like JVM.

Also removed Job to make it consistent with JVM.

[Thomas-Vos]

Maybe SelectorManager should use IO by default on supported platforms, or document that IO should be passed.

ktor/ktor-network/common/src/io/ktor/network/selector/SelectorManagerCommon.kt

Lines 16 to 18 in bd652e4

	public expect fun SelectorManager(
	dispatcher: CoroutineContext = EmptyCoroutineContext
	): SelectorManager

[Thomas-Vos]

Still have to check why the testEchoByteArray[macosX64] failed. Not sure whether this is caused by the changes in this PR or if the issue was already present. Cannot reproduce locally, currently unsure how this failure is even possible.

https://ktor.teamcity.com/buildConfiguration/Ktor_KtorMatrixNativeMacOSX64/319909

[Thomas-Vos]

Test on CI is no longer failing (two runs), so #4680 fixed it or the test is flaky.