Fix Generate (timed) crash upon disposing a long sequence

[akarnokd]

Due to how recursive scheduling has to link up IDisposables in the UserWorkItem class, a long running timed Generate would produce a long chain of UserWorkItems and crash with stack overflow if the whole sequence itself is disposed.

The fix is to have the recursive scheduling in Generate not return the next step's IDisposable from the function but keep replacing a main reference with the newer and newer IDisposables, and thus no link chain is formed.

The use of the SAD indirection is necessary because when the Schedule returns with the IDisposable, the payload may have already run and we would overwrite a newer step's IDisposable.

[khellang]

Thanks for fixing this memory leak. This is what my app looked like with 4.1.0:

And this is what it looks like now (with 4.3.0-preview.53):

Any clue when this will be part of an official release?

[khellang]

I left the app running over night with the newly released 4.3.1 release and this is the result:

Thanks again for fixing this and cutting a release so swiftly 😘