expand missing scheduler argument

[benlesh]

I pointed out here, that we are missing a scheduler argument for expand. This is necessary for changing the behavior of when expand subscribes to observables returned by the selector.

It should default to no/null/undefined scheduler, which means immediate execution.

Related to #766 #788 #793

[luisgabriel]

I can work on this one.

[kwonoj]

@luisgabriel 👍

[lock]

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