Fixed DelegateProxy to check if it's running on main thread

[purple-x]

Addressing issue: #1874

[RxPullRequestBot]

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[kzaher]

Hi @ryu1006 ,

could you please take a look at Linux unit tests. I would like to release this with a patch release this weekend.

Otherwise looks ok to me.

@freak4pc Could you please smoke test this in a real app. I don't currently have access to any bigger codebase.

[freak4pc]

@kzaher I'll give it a whirl and let you know.

[purple-x]

@kzaher checked and saw that isMainThread is not implemented in Linux's Foundation. Fortunately enough, majority of the instances for ensureRunningOnMainThread is only on files that ensure it's not running on Linux or is in Debug. Only VirtualTimeScheduler does not have these checks, so reverted it to the previous ensureExecutingOnScheduler function.

[freak4pc]

@kzaher I've run ~500 unit tests of a larger codebase and everything seems to run smoothly.
I guess the main pieces we have counting on DelegateProxy is RxDataSources - and that seems to work and pass tests with no issues.