[twitter-server] Stream admin/events page

Problem

The Sink exposes an iterator over Events, which we can render into an
HTML stream (we already stream for the JSON handler), but we don't do
this for the HTML admin page. Consequently, this requires the controller
to fully buffer the HTML which could be potentially large.

Solution

Incrementally render and write HTML as we iterate over Events in the
Sink.

RB_ID=608234

util-core/src/main/scala/com/twitter/concurrent/Spool.scala

@@ -295,6 +295,21 @@ object Spool {
     }
   }
 
+  /**
+   * Lazily builds a Spool from a Seq.
+   *
+   * The main difference between this and `seqToSpool` is that this method also
+   * consumes the Seq lazily, which means if used with Streams, it will
+   * preserve laziness.
+   */
+  def fromSeq[A](seq: Seq[A]): Spool[A] = {
+    def go(as: Seq[A]): Future[Spool[A]] =
+      if (as.isEmpty) Future.value(Spool.empty)
+      else Future.value(as.head *:: go(as.tail))
+
+    if (seq.isEmpty) Spool.empty else seq.head *:: go(seq.tail)
+  }
+
   /**
    * Adds an implicit method to efficiently convert a Seq[A] to a Spool[A]
    */