Shave O(500ms) off of sbt startup

It turns out that it can take roughly one second to instantiate a
scala.nsc.tools.Global instance for the first time. When sbt is starting
up, it also takes nearly 2 seconds to initialize logging. We can speed
up the boot time by doing these two things concurrently. On my machine,
I saw on average a 500ms decrease in startup time after this change.

main/src/main/scala/sbt/Main.scala

@@ -44,6 +44,14 @@ final class xMain extends xsbti.AppMain {
     val instance = clazz.getField("MODULE$").get(null)
     val runMethod = clazz.getMethod("run", classOf[xsbti.AppConfiguration])
     try {
+      new Thread("sbt-load-global-instance") {
+        setDaemon(true)
+        override def run(): Unit = {
+          // This preloads the scala.tools.nsc.Global as a performance optimization"
+          loader.loadClass("sbt.internal.parser.SbtParser$").getField("MODULE$").get(null)
+          ()
+        }
+      }.start()
       runMethod.invoke(instance, modifiedConfiguration).asInstanceOf[xsbti.MainResult]
     } catch {
       case e: InvocationTargetException =>

main/src/main/scala/sbt/internal/parser/SbtParser.scala

@@ -108,7 +108,7 @@ private[sbt] object SbtParser {
   private[sbt] final val globalReporter = new UniqueParserReporter
   private[sbt] var scalacGlobalInitReporter: Option[ConsoleReporter] = None
 
-  private[sbt] final lazy val defaultGlobalForParser = {
+  private[sbt] final val defaultGlobalForParser = {
     val options = "-cp" :: s"$defaultClasspath" :: "-Yrangepos" :: Nil
     val reportError = (msg: String) => System.err.println(msg)
     val command = new CompilerCommand(options, reportError)