IMPALA-843: Do not close reader contexts until plan fragment close

Fixes a crash that occurs in some cases when io buffers are still used and
child nodes are closed early. We close child nodes early when all rows have
been consumed and resources are transfered, but in some cases io buffers are
still in use when a scan node is closed. We avoid this problem by only
closing reader contexts when the entire fragment is closed.

Change-Id: Ie62cdecdcd530bdc61dd4e83cd9ecfc7d2c93ef6
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1806
Reviewed-by: Matthew Jacobs <mj@cloudera.com>
Tested-by: jenkins