[SPARK-4386] Improve performance when writing Parquet files.

If you profile the writing of a Parquet file, the single worst time consuming call inside of org.apache.spark.sql.parquet.MutableRowWriteSupport.write is actually in the scala.collection.AbstractSequence.size call. This is because the size call actually ends up COUNTING the elements in a scala.collection.LinearSeqOptimized.length ("optimized?").

This doesn't need to be done. "size" is called repeatedly where needed rather than called once at the top of the method and stored in a 'val'.