Eliminate unneeded LimitedDispatcher instances on Dispatchers.Default and Dispatchers.IO (#3562)

* Handle `Dispatchers.IO.limitedParallelism(Int.MAX_VALUE)` case

`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 eliminating the `LimitedDispatcher`
instance and its queue in this extreme case.

* Handle `Dispatchers.Default.limitedParallelism` when requested parallelism >= core pool size (#3442)

`LimitedDispatcher.limitedParallelism` returns `this` if requested parallelism is greater or equal
to the own parallelism of the said `LimitedDispatcher`. `DefaultScheduler` has parallelism effectively set
to `CORE_POOL_SIZE`.

Before the change `LimitedDispatcher(parallelism)` was returned. While it does work as expected, any submitted task
goes through its queue and `parallelism` number of workers. The change allows eliminating the `LimitedDispatcher`
instance and its queue in case the requested parallelism is greater or equal to `CORE_POOL_SIZE`.

Fixes #3442

Author: dovchinnikov

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

@@ -14,6 +14,14 @@ internal object DefaultScheduler : SchedulerCoroutineDispatcher(
     CORE_POOL_SIZE, MAX_POOL_SIZE,
     IDLE_WORKER_KEEP_ALIVE_NS, DEFAULT_SCHEDULER_NAME
 ) {
+
+    @ExperimentalCoroutinesApi
+    override fun limitedParallelism(parallelism: Int): CoroutineDispatcher {
+        parallelism.checkParallelism()
+        if (parallelism >= CORE_POOL_SIZE) return this
+        return super.limitedParallelism(parallelism)
+    }
+
     // Shuts down the dispatcher, used only by Dispatchers.shutdown()
     internal fun shutdown() {
         super.close()
@@ -38,6 +46,13 @@ 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 >= MAX_POOL_SIZE) return this
+        return super.limitedParallelism(parallelism)
+    }
 }
 
 // Dispatchers.IO