Possible memory leak with test data

I use the code below to add 200,000 tuples to a test relvar. After execution, `project-m36-server` uses about 1GB of RAM. Once I quit the server and start it again, it uses about 2.4GB of RAM.

(Sorry about the code wall, I moved everything relevant to the same file for ease of test.)

Test repo is [here](https://github.com/farzadbekran/leak-test)

To reproduce, start ghci in the DB module, load it and run:
```
> genTestData
Right ()
> getTestData
Right 100
```
After that if you check, you'll see that `project-m36-server` is using about 1GB of RAM
If you close the server and start it again, it will be using about 2.4GB