Fix #6698: Synchronize all RPC messages with systemOut notifications

`systemOut` notifications are buffered so that they are sent at most
once every 20 millisecond. Other RPC messages are not buffered.
As a consequence, some RPC messages can pass in front of some
systemOut notifications.
That's why `sbt --client run` can exit before it receives all the logs.

In general I think it is safer to maintain the order of all messages.
To do so we can force the flush of systemOut before each RPC message.

main/src/main/scala/sbt/internal/server/NetworkChannel.scala

@@ -602,6 +602,7 @@ final class NetworkChannel(
       execId: String,
       err: JsonRpcResponseError
   ): Unit = {
+    forceFlush()
     val m = JsonRpcResponseMessage("2.0", execId, None, Option(err))
     val bytes = Serialization.serializeResponseMessage(m)
     publishBytes(bytes)
@@ -611,13 +612,17 @@ final class NetworkChannel(
   private[sbt] def jsonRpcNotify[A: JsonFormat](method: String, params: A): Unit = {
     val m =
       JsonRpcNotificationMessage("2.0", method, Option(Converter.toJson[A](params).get))
-    if (method != Serialization.systemOut) log.debug(s"jsonRpcNotify: $m")
+    if (method != Serialization.systemOut) {
+      forceFlush()
+      log.debug(s"jsonRpcNotify: $m")
+    }
     val bytes = Serialization.serializeNotificationMessage(m)
     publishBytes(bytes)
   }
 
   /** Notify to Language Server's client. */
   private[sbt] def jsonRpcRequest[A: JsonFormat](id: String, method: String, params: A): Unit = {
+    forceFlush()
     val m =
       JsonRpcRequestMessage("2.0", id, method, Option(Converter.toJson[A](params).get))
     log.debug(s"jsonRpcRequest: $m")
@@ -654,7 +659,13 @@ final class NetworkChannel(
 
   import scala.collection.JavaConverters._
   private[this] val outputBuffer = new LinkedBlockingQueue[Byte]
-  private[this] val flushFuture = new AtomicReference[java.util.concurrent.Future[_]]
+  private[this] val flushExecutor = Executors.newSingleThreadScheduledExecutor(
+    r => new Thread(r, s"$name-output-buffer-timer-thread")
+  )
+  private[this] def forceFlush() = {
+    Util.ignoreResult(flushExecutor.shutdownNow())
+    doFlush()
+  }
   private[this] def doFlush()() = {
     val list = new java.util.ArrayList[Byte]
     outputBuffer.synchronized(outputBuffer.drainTo(list))
@@ -673,13 +684,9 @@ final class NetworkChannel(
      * probably long enough to catch each burst but short enough to not introduce
      * noticeable latency.
      */
-    private[this] val executor =
+    private[this] val flushFuture = new AtomicReference[java.util.concurrent.Future[_]]
-      Executors.newSingleThreadScheduledExecutor(
-        r => new Thread(r, s"$name-output-buffer-timer-thread")
-      )
     override def close(): Unit = {
-      Util.ignoreResult(executor.shutdownNow())
+      forceFlush()
-      doFlush()
     }
     override def write(b: Int): Unit = outputBuffer.synchronized {
       outputBuffer.put(b.toByte)
@@ -689,7 +696,7 @@ final class NetworkChannel(
         case null =>
           try {
             flushFuture.set(
-              executor.schedule(
+              flushExecutor.schedule(
                 (() => {
                   flushFuture.set(null)
                   doFlush()