IMPALA-1471: Bug in spilling of PHJ that was affecting left anti and outer joins.

In cases where we had to spill the probe side of PHJs, we were not only appending
the probe row to the tuple stream to be spilled, but we were also getting into the
regular processing loop with the iterator set to End(). In the case of left anti
and left outer joins, the result was to incorrectly output this row, since it did not
have a match.

This bug had a small perf impact for all spilling joins because we were doing an
unnecessary loop for each probe row we had to spill.

This patch solves the problem by immediately going to the next probe row if the
current row is spilled. Additionally, it fixes a bug in the block mgr where there
was a code path we were not counting correctly the number of pinned buffers.
It also adds tpch-q21 in the set of queries to run in the spilling test.

Change-Id: I762f5c41fe468e4485a4b31dabe2e53f6b49ae24
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/5313
Reviewed-by: Ippokratis Pandis <ipandis@cloudera.com>
Tested-by: jenkins