[WIP] Rework Scheduler

[mbonneau]

This PR returns the EventLoopScheduler to the v1 style construction.

This also adds AsyncSchedulerInterface for operators and observables that require async schedulers.

The README.md still needs to be updated.

[coveralls]

Coverage remained the same at 100.0% when pulling 7c058d5 on ReactiveX:2.x.

[martinsik]

I'm just curious what happened to Loop and async-interop. I read the discussions in async-interop/event-loop repo and it seems the API is likely to change and it'll take some for the first stable release. Is this the reason we're going back to React\EventLoop?

[mbonneau]

@martinsik This change is being made because version 2 of RxPHP is taking too long to release with the dependency on the interop loop as the only blocking issue.

When first developing version 2 of RxPHP, it appeared that work on the Async-Interop Loop library had settled a bit and we had hoped that a stable version was close (see: async-interop/event-loop#119). We chose to try and push things forward with adoption of the interop loop standard. Since then, there have been more issues raised with the interop loop and consensus is taking a bit longer than we would like for inclusion into this next release of RxPHP.

Inclusion of the interop loop would have allowed users to remove a small bit of boiler plate code. With this PR, construction of the actual scheduler will be the same as it was in v1 and then you can set the default scheduler in the Scheduler static. RxPHP is loop agnostic (react is a dev-dependency for testing).

[coveralls]

Coverage remained the same at 100.0% when pulling 7c058d5 on ReactiveX:2.x.

[davidwdan]

I'm going to merge this because the async interop loop project is all but dead. If and when there is a loop standard we can introduce new interop loop scheduler under 2.x and then bump the version to v3 or what ever the next major is, when we're reading to make it the default.