[split] util-core: Create daemon threads in all stock `com.twitter.ut…

…il.FuturePool`s

util-core's default `ExecutorService`s create non-daemon threads, which don't
play nicely with graceful shutdown functionality. This change forces all stock
FuturePool implementations to spawn daemon threads. All threads created by
FuturePools will be daemonized, which will improve the graceful shutdown case.

RB_ID=296659

util-core/src/main/scala/com/twitter/util/FuturePool.scala

@@ -40,7 +40,7 @@ object FuturePool {
   }
 
   private lazy val defaultExecutor = Executors.newCachedThreadPool(
-    new NamedPoolThreadFactory("UnboundedFuturePool")
+    new NamedPoolThreadFactory("UnboundedFuturePool", makeDaemons = true)
   )
 
   /**