[SPARK-15762][SQL] Cache Metadata & StructType hashCodes; use singleton Metadata.empty

We should cache `Metadata.hashCode` and use a singleton for `Metadata.empty` because calculating metadata hashCodes appears to be a bottleneck for certain workloads.

We should also cache `StructType.hashCode`.

In an optimizer stress-test benchmark run by ericl, these `hashCode` calls accounted for roughly 40% of the total CPU time and this bottleneck was completely eliminated by the caching added by this patch.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #13504 from JoshRosen/metadata-fix.

sql/catalyst/src/main/scala/org/apache/spark/sql/types/Metadata.scala

@@ -104,7 +104,8 @@ sealed class Metadata private[types] (private[types] val map: Map[String, Any])
     }
   }
 
-  override def hashCode: Int = Metadata.hash(this)
+  private lazy val _hashCode: Int = Metadata.hash(this)
+  override def hashCode: Int = _hashCode
 
   private def get[T](key: String): T = {
     map(key).asInstanceOf[T]
@@ -115,8 +116,10 @@ sealed class Metadata private[types] (private[types] val map: Map[String, Any])
 
 object Metadata {
 
+  private[this] val _empty = new Metadata(Map.empty)
+
   /** Returns an empty Metadata. */
-  def empty: Metadata = new Metadata(Map.empty)
+  def empty: Metadata = _empty
 
   /** Creates a Metadata instance from JSON. */
   def fromJson(json: String): Metadata = {

sql/catalyst/src/main/scala/org/apache/spark/sql/types/StructType.scala

@@ -112,7 +112,8 @@ case class StructType(fields: Array[StructField]) extends DataType with Seq[Stru
     }
   }
 
-  override def hashCode(): Int = java.util.Arrays.hashCode(fields.asInstanceOf[Array[AnyRef]])
+  private lazy val _hashCode: Int = java.util.Arrays.hashCode(fields.asInstanceOf[Array[AnyRef]])
+  override def hashCode(): Int = _hashCode
 
   /**
    * Creates a new [[StructType]] by adding a new field.