IMPALA-3964: Fix crash when a count(*) is performed on a nested collection.

The Bug: Prior to this patch, a DCHECK was used to verify that the
underlying memory pool for the scratch batch was empty in a count based
scenario. For IMPALA-3964 (where a count(*) is performed on a nested
collection), if a Parquet column chunk is compressed, upon reading each
new data page it would be decompressed and eventually placed in to the
underlying scratch batch memory pool causing the aforementioned DCHECK
to fail. This was not picked up in the test suite as the TPCH nested
Parquet data is not compressed.

The Fix: Removed the erroneous DCHECK. Added logic to determine if any
memory in the scratch batch needs to be freed (due to the transfer that
occurs from the decompressed data pool), if so, it will be done.
Augmented the load_nested.py script to snappy compress each of the
tables within the 'tpch_nested_parquet' database. This is consistent with
how the flat TPCH Parquet data set is stored. Regarding test coverage,
there are already a number of tests that will perform nested collection
counts against the tables in the 'tpch_nested_parquet' database. For
uncompressed nested Parquet, the 'test_nested_types.py' test suite
leverages the 'ComplexTypesTbl' table to provide good coverage.

Change-Id: Id0955c85d18dfba4bd29a35ec95d0355da050607
Reviewed-on: http://gerrit.cloudera.org:8080/3940
Reviewed-by: Michael Ho <kwho@cloudera.com>
Reviewed-by: Dan Hecht <dhecht@cloudera.com>
Tested-by: Internal Jenkins

testdata/bin/load_nested.py

@@ -257,6 +257,7 @@ def load():
 
         CREATE TABLE customer
         STORED AS PARQUET
+        TBLPROPERTIES('parquet.compression'='SNAPPY')
         AS SELECT * FROM tmp_customer;
 
         DROP TABLE tmp_orders_string;
@@ -265,6 +266,7 @@ def load():
 
         CREATE TABLE region
         STORED AS PARQUET
+        TBLPROPERTIES('parquet.compression'='SNAPPY')
         AS SELECT * FROM tmp_region;
 
         DROP TABLE tmp_region_string;
@@ -272,6 +274,7 @@ def load():
 
         CREATE TABLE supplier
         STORED AS PARQUET
+        TBLPROPERTIES('parquet.compression'='SNAPPY')
         AS SELECT * FROM tmp_supplier;
 
         DROP TABLE tmp_supplier;