IMPALA-1012: lower the default parquet file size to 256 MB

Parquet files cannot be split into multiple scan ranges.  So, the
advantage of smaller files is more potential parallelism for queries.
Also, less memory is used to write smaller files.

However, we don't want to go too small, or we could suffer on the I/O
performance during a scan. To verify that 256 MB is not too small, I ran
some scan heavy queries on the 10-node cluster using the 10000 GB TPCH
lineitem table. The baseline is 1 GB files, and even at that size the
cluster is small enough that there should be plenty of parallalism.  So,
the experiment is to determine whether scans (and in particular I/O
efficiency) suffers at smaller file size.  Results (best, worse
execution times in seconds over four runs):

(a) select l_comment from lineitem where l_comment is null

1 GB files:    157.05, 191.42
256 MB files:  129.51, 132.29
64 MB files:   127.60, 148.79

(b) select l_orderkey from lineitem where l_orderkey is null;

1 GB files:    34.27,  41.97
256 MB files:  35.30,  38.86
64 MB files:   42.86,  61.03

Also, I took a look at the profiles and nothing bad stands out at 256
MB.  So, 256 MB looks like a good compromise.

Change-Id: Ifa8945d300141b2b2bbf66a3f8ce02644ddf14b3
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4519
Reviewed-by: Skye Wanderman-Milne <skye@cloudera.com>
Tested-by: jenkins