Allow TaskManagers to manage TaskManagers

[qstokkink]

Regarding the Tribler code, I see several instances of "something with a TaskManager" that manages "something with a TaskManager".

The procedure for managing these things is:

1. Create a child ("something with a TaskManager") in the parent ("something with a TaskManager") code.
2. Wait for shutdown.
3. Destroy the children.

Each child waits for shutdown (by the parent) itself. However, if there is a link with the parent, it needs to make sure that the parent has not shut down yet.

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My proposal is to create a TaskManager.register_taskmanager() that simply shuts down the given task manager on shutdown. This avoids the above explicit management by downstream code bases.

[qstokkink]

I think we can include this in the IPv8 3.0 release.