IMPALA-1046: Fix race in io mgr with cancelled readers and OOM.

Change-Id: I3bce1a0444bd40528a474fb53c3af4d2b53b4aa7
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3022
Reviewed-by: Srinath Shankar <sshankar@cloudera.com>
Tested-by: jenkins