Rowbatch.cc uses LZ4 codec instead of Snappy codec

Comparison of Exchange node data for Lz4 and Snappy
running query: select (star symbol) from tpch.lineitem order by
 l_orderkey

Snappy:
XCHANGE_NODE (id equal 2):(Total: 36s021ms...)
BytesReceived: 26.75 MB (28047762)
DeserializeRowBatchTimer: 246.561ms

Lz4:
EXCHANGE_NODE (id equal 2):(Total: 34s699ms...)
BytesReceived: 11.20 MB (11741118)
DeserializeRowBatchTimer: 131.379ms

Change-Id: Iae8d212ba0fd508542f3ef9ddaf7507426e13253
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3120
Reviewed-by: Paden Tomasello <paden.tomasello@cloudera.com>
Tested-by: jenkins
Add partition pruning tests

The following changes are included in this commit:
1. Modified the alltypesagg table to include an additional partition key
that has nulls.
2. Added a number of tests in hdfs.test that exercise the partition
pruning logic (see IMPALA-887).
3. Modified all the tests that are affected by the change in alltypesagg.

Change-Id: I1a769375aaa71273341522eb94490ba5e4c6f00d
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2874
Reviewed-by: Dimitris Tsirogiannis <dtsirogiannis@cloudera.com>
Tested-by: jenkins