MB-9769: Test case where item_pager hangs

- Noticed that in test_item_pager testcase,
sometimes the chunk of data removed makes mem_used
just a little lesser than the high_water_mark,
in this case mem_used never goes all the way down
to being lesser than or equal to the low_wat_mark.

- Fix ensures that in the above mentioned case
whenever the mem_used crosses the high_wat_mark,
mem_used always comes down to being lesser than or
equal to the low_wat_mark.

Change-Id: I61ceb94bdc59909c40881785e48d500c5dd98e99
Reviewed-on: http://review.couchbase.org/31310
Reviewed-by: Sundararaman Sridharan <sundar.sridharan@gmail.com>
Tested-by: Sundararaman Sridharan <sundar.sridharan@gmail.com>