Avoid memory leak in loaned-message return path

[tobiasstarkwayve]

See #85

This PR fixes a memory leak on the code path returning loaned messsages to the middleware.

[tobiasstarkwayve]

@m-dahl Here is the patch I promised back in March (sorry, I got sidetracked in the meantime and only now had the time to come back to this).

I have roughly tested it on our Wayve-internal build of this, but not through the cargo path. I tried using the Dockerfiles in tests/, but wasn't able to get the build to run as it was looking for an "OUT_FILES" environment variable. Is there more documentation on how to run these? If not, could you double-check the changes and check that they work?

[m-dahl]

Thanks! Sorry for the delay.

The easiest way to run the CI pipeline is either on github (next time it will run automatically for you since you will have a commit in the history when I have merged this), or you can use act to run them locally.