handle Dispatchers.IO.limitedParallelism(Int.MAX_VALUE) case (Kotlin#3442)

`LimitedDispatcher.limitedParallelism` returns `this` if requested parallelism is greater or equal
to the own parallelism of the said `LimitedDispatcher`. `UnlimitedIoScheduler` has parallelism effectively set
to `Int.MAX_VALUE`, so `parallelism >= this.parallelism` check folds into `parallelism == Int.MAX_VALUE`.

Before the change `LimitedDispatcher(Int.MAX_VALUE)` was returned. While it does work as expected, any submitted task
goes through its queue and `Int.MAX_VALUE` number of workers. The change allows to eliminate the `LimitedDispatcher`
instance and its queue in this extreme case.

Author: dovchinnikov

kotlinx-coroutines-core/jvm/src/scheduling/Dispatcher.kt

@@ -38,6 +38,13 @@ private object UnlimitedIoScheduler : CoroutineDispatcher() {
     override fun dispatch(context: CoroutineContext, block: Runnable) {
         DefaultScheduler.dispatchWithContext(block, BlockingContext, false)
     }
+
+    @ExperimentalCoroutinesApi
+    override fun limitedParallelism(parallelism: Int): CoroutineDispatcher {
+        parallelism.checkParallelism()
+        if (parallelism == Int.MAX_VALUE) return this
+        return super.limitedParallelism(parallelism)
+    }
 }
 
 // Dispatchers.IO