Loop as long as the warmup thread is running

And check for the number of items read back in after the
warmup thread has completed.

Change-Id: I2e2cfc17ec0a25be0e29ff9405fc62de9d211cf1
Reviewed-on: http://review.couchbase.org/40684
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>