[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

scrooge-core/src/main/scala/com/twitter/scrooge/internal/TProtocols.scala

@@ -3,6 +3,7 @@ package com.twitter.scrooge.internal
 import com.twitter.scrooge.TFieldBlob
 import com.twitter.scrooge.ThriftEnum
 import com.twitter.scrooge.ThriftUnion
+import com.twitter.util.MapUtil
 import java.nio.ByteBuffer
 import java.util.function.ObjDoubleConsumer
 import java.util.function.ObjLongConsumer
@@ -68,18 +69,19 @@ final class TProtocols private[TProtocols] {
     readValue: TProtocol => V
   ): collection.Map[K, V] = {
     val tmap = protocol.readMapBegin()
-    if (tmap.size == 0) {
+    val size = tmap.size
+    if (size == 0) {
       protocol.readMapEnd()
       Map.empty[K, V]
     } else {
-      val map = new mutable.HashMap[K, V]()
+      val map = MapUtil.newHashMap[K, V](size)
       var i = 0
       do {
         val key = readKey(protocol)
         val value = readValue(protocol)
         map(key) = value
         i += 1
-      } while (i < tmap.size)
+      } while (i < size)
       protocol.readMapEnd()
       map
     }