refactor(subscribers): reduce inheritence usage in subscribers

[benlesh]

We have a few operators that have gotten a little deep with inheritance. In particular switchLatest (aka flatMapLatest), in which

SwitchLatestSubscriber extends FlatMapSubscriber
FlatMapSubscriber extends MergeSubscriber
and MergeSubscriber extends Subscriber

The problems with this are:

• It gets a little hard to follow after a bit.
• refactoring becomes more difficult
• less modularity

If shared functionality/methods are a goal, perhaps it would be better to share them via module imports?

attn @trxcllnt

[benlesh]

It's my opinion that we should probably keep inheritance to one level deep, with the occasional exception.

[benlesh]

I think this has been met to some degree by #316. Closing for now.