Memory leaks while integration and e2e testing

Intended outcome: integration and e2e tests using `apollo-server` should cleanly execute and clean up all memory.

Actual outcome: integration and e2e tests using `apollo-server` suffer from memory leaks which inflate test run times

How to reproduce the issue:

1. `git clone git@github.com:apollographql/fullstack-tutorial.git`
2. `cd fullstack-tutorial/final/server`
3. `npm install && npm install weak`
4. `npx jest --detectLeaks`
5. Observe that the integration and e2e tests result in memory leaks. The memory leaks only occur after making a request to the server during the test.

Version in use:
`apollo-server@2.6.1`

Reproduce the leak from fullstack-tutorial/final/server with npm install && npm install weak and npx jest --detectLeaks. Compare runs before and after a test request to apollo-server, then trace the server-test cleanup path. Done means the integration and e2e tests cleanly release memory without inflated runtimes.

Written by the indexing model from the issue text.