Clean up resources after worker thread is terminated

[maaquib]

Description of changes:

• Terminate err and out ReaderThreads after its corresponding WorkerThread has been terminated
• Clean up FDs associated with a terminated WorkerThread
• Pins pytest version in CI test to avoid error fixture is being applied more than once to the same function

Testing done (Ubuntu18.04):

• Stress tested for 450000 iterations of model load and unload
• CI tests successful

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.