ml/params: cache stringified versions of Params

repeated lookup of paramter values within ParamMaps was causing
a significant (35-45%) performance hit within LogisticRegression
(SPARK-13132) due to the string interpolation performed by every
call to hashCode.

cache the stringified representation of the Param in a private
instance variable, so that the string interpolation only happens
once

mllib/src/main/scala/org/apache/spark/ml/param/params.scala

@@ -117,7 +117,9 @@ class Param[T](val parent: String, val name: String, val doc: String, val isVali
     }
   }
 
-  override final def toString: String = s"${parent}__$name"
+  private[this] val stringRepresentation = s"${parent}__$name"
+
+  override final def toString: String = stringRepresentation
 
   override final def hashCode: Int = toString.##