[finagle-netty4] Expose a knob to control io-ratio in EpollEventLoop

Problem
We can't control how much time netty spends processing i/o events compared to other work

Solution
Introduce a knob that allows us to do this

Differential Revision: https://phabricator.twitter.biz/D1175405

finagle-netty4/src/main/scala/com/twitter/finagle/netty4/WorkerEventLoop.scala

@@ -61,19 +61,27 @@ private object WorkerEventLoop {
     )
   }
 
-  def make(executor: Executor, numWorkers: Int): EventLoopGroup = {
+  def make(executor: Executor, numWorkers: Int, ioRatio: Int = 50): EventLoopGroup = {
     workerPoolSize.addAndGet(numWorkers)
     val result =
-      if (useNativeEpoll() && Epoll.isAvailable) new EpollEventLoopGroup(numWorkers, executor)
-      else new NioEventLoopGroup(numWorkers, executor)
+      if (useNativeEpoll() && Epoll.isAvailable) {
+        val group = new EpollEventLoopGroup(numWorkers, executor)
+        group.setIoRatio(ioRatio)
+        group
+      } else {
+        new NioEventLoopGroup(numWorkers, executor)
+      }
 
     eventLoopGroups.add(result)
 
     result
   }
 
   lazy val Global: EventLoopGroup = make(
-    Executors.newCachedThreadPool(new BlockingTimeTrackingThreadFactory(mkNettyThreadFactory())),
-    numWorkers()
+    executor = Executors.newCachedThreadPool(
+      new BlockingTimeTrackingThreadFactory(mkNettyThreadFactory())
+    ),
+    numWorkers = numWorkers(),
+    ioRatio = ioRatio()
   )
 }

finagle-netty4/src/main/scala/com/twitter/finagle/netty4/ioRatio.scala

@@ -0,0 +1,21 @@
+package com.twitter.finagle.netty4
+
+import com.twitter.app.GlobalFlag
+
+/**
+ * Flag for defining the percentage of the desired amount of time spent for I/O in the child event
+ * loops. The default value is 50, which means the event loop will try to spend the same amount of
+ * time for I/O as for non-I/O tasks.
+ *
+ * {{
+ *  -com.twitter.finagle.netty4.ioRatio=50
+ * }}
+ *
+ */
+object ioRatio
+    extends GlobalFlag(
+      50,
+      "Sets the percentage of the desired amount of time spent for I/O in the child event loops. " +
+        "The default value is 50, which means the event loop will try to spend the same amount of " +
+        "time for I/O as for non-I/O tasks."
+    )