IMPALA-1069: Cap the maximum number of io buffers the iomgr will hold onto.

Change-Id: I20704e4b10a3c4e908c96577dc65c01cceaf30c4
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/3713
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: jenkins