EventLoopScheduler: set _nextItem to null on Tick

[ghost]

This fixes a potential memory leak.

If _queue is empty after the item is removed from it, then _nextItem is never overriden and therefore keeps a reference to item.
Until another action is scheduled with a future due time, the item and all its associated state is kept in memory.

[dnfclas]

Hi @TobBrandt-Work, I'm your friendly neighborhood .NET Foundation Pull Request Bot (You can call me DNFBOT). Thanks for your contribution!

This seems like a small (but important) contribution, so no Contribution License Agreement is required at this point. Real humans will now evaluate your PR.

TTYL, DNFBOT;

[bartdesmet]

@TobBrandt-Work, thanks for your contribution. We're looking into adding this to Rx 4.0 but there seems to be a merge conflict. If you have time to help to resolve this, feel free to do so. If you don't, that's fine too, and we'll have a look over here. The CI issue should have been resolved by now, so once the merge conflict goes away, I'd expect everything to turn green.

[ghost]

I have merged in the changes from upstream.

[clairernovotny]

@TobBrandt-Work Can you please rebase this off of develop? It should not be going into master.

Thanks

[ghost]

I cannot change the source branch of the PR (at least I don't know how).

New PR: #421