handle Dispatchers.Default.limitedParallelism when requested parallelism >= core pool size (Kotlin#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 to eliminate the `LimitedDispatcher`
instance and its queue in case the requested parallelism is greater or equal to `CORE_POOL_SIZE`.

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()