[scrooge] Preallocate hashmaps during deserialization

Problem
When a map is deserialized in scrooge we create it with default capacity, which means that for larger maps we need to resize it multiple times and each time recalculate key hashes

Solution
Allocate a map with sufficient capacity from the start.

Differential Revision: https://phabricator.twitter.biz/D1174445

util-core/src/main/scala-2.12-/com/twitter/util/MapUtil.scala

@@ -0,0 +1,13 @@
+package com.twitter.util
+
+import scala.collection.mutable
+
+object MapUtil {
+
+  def newHashMap[K, V](initialCapacity: Int, loadFactor: Double = 0.75): mutable.HashMap[K, V] = {
+    new mutable.HashMap[K, V]() {
+      this._loadFactor = (loadFactor * 1000).toInt
+      override protected val initialSize: Int = (size.toLong / loadFactor).toInt
+    }
+  }
+}

util-core/src/main/scala-2.13+/com/twitter/util/MapUtil.scala

@@ -0,0 +1,10 @@
+package com.twitter.util
+
+import scala.collection.mutable
+
+object MapUtil {
+
+  def newHashMap[K, V](initialCapacity: Int, loadFactor: Double = 0.75): mutable.HashMap[K, V] = {
+    new mutable.HashMap[K, V](initialCapacity, loadFactor)
+  }
+}