Don't do progress work on the main thread

I noticed that no-op compile was slower in
https://github.com/sbt/sbt/issues/5508 using 1.4.0-RC2 than 1.4.0-RC1.
It took around 400ms with 1.4.0-RC2 and 200-250ms on RC1. Git bisect
brought me to 41afe9fbdb which I
remembered I'd been slightly concerned about from a performance
perspective but didn't get around to testing. The problem is that we
were blocking the task from running while determing whether or not we
should force a progress report. We can do that work on the background
thread instead so the task can begin running immediately.

main/src/main/scala/sbt/internal/TaskProgress.scala

@@ -107,10 +107,15 @@ private[sbt] class TaskProgress(
 
   override def afterReady(task: Task[_]): Unit =
     if (!closed.get) {
-      if (skipReportTasks.contains(getShortName(task))) {
-        lastTaskCount.set(-1) // force a report for remote clients
-        report()
-      } else Util.ignoreResult(active.put(task, schedule(threshold, recurring = false)(doReport())))
+      try {
+        Util.ignoreResult(executor.submit((() => {
+          if (skipReportTasks.contains(getShortName(task))) {
+            lastTaskCount.set(-1) // force a report for remote clients
+            report()
+          } else
+            Util.ignoreResult(active.put(task, schedule(threshold, recurring = false)(doReport())))
+        }): Runnable))
+      } catch { case _: RejectedExecutionException => }
     } else {
       logger.debug(s"called afterReady for ${taskName(task)} after task progress was closed")
     }