Memory leak when using @Async after upgrading from 5.1 to 5.2
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**Affects:** 5.2.0.RC1

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In our application I upgraded from spring-framework 5.1.9 to 5.2.0.RC1 and our tests started leaking memory.

Previously it looked like this in VisualVM:

![image](https://user-images.githubusercontent.com/109243/64158877-8d43cf80-ce39-11e9-8582-85357e439198.png)

And now it looks like this:

![image](https://user-images.githubusercontent.com/109243/64158907-9b91eb80-ce39-11e9-963a-e2c08716f55a.png)

I've made a heapdump to see where the memory is going:

![image](https://user-images.githubusercontent.com/109243/64163425-8325cf00-ce41-11e9-922d-886c6feac4e4.png)

I've seen something similar in our application previously, but that was solved by using `@MockBean` instead of a lot of `@ContextConfiguration` with `@Configuration` classes inside our test classes to override beans with mocked objects :thinking:...