Pillowfight memory leak

What's the issue?

When we set the `--num-items` option to a large number something like 150000000 after pillowfight populates those documents and continues to modify them it slowly uses up more and more memory until potentially it can use up almost all of the memory available to it on the system.

Reproduction steps:

1. Set the `--num-items` to 150000000, the `-m` option to 8000 and `-M` option to 10000. The command should look something like this: `./cbc-pillowfight -R -M 10000 -m 8000 -I 150000000 -J -t6 -U '<cluster-url>' -u <username> -P <password>`
2. After population is done and modification of documents ensues we should observe memory usage increase over time.

What's the fix?

Investigate why the memory is being used up in this scenario and come up with a fix.