Subscription overhaul2

[akarnokd]

Renamed and changed things according to comments in #603

[cloudbees-pull-request-builder]

RxJava-pull-requests #554 SUCCESS
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[headinthebox]

I like to keep the number of different Subscriptions as small as possible

What is the use-case for ValuedCompositeSubscription?
What is the use-case SafeObservableSubscription?

[akarnokd]

Use case for ValuedCompositeSubscription: associate observers with their subscriptions.

ValuedCompositeSubscription<Observable<? super T>> observers = ...

but I usually ended up using regular map and locks anyway. So it is not that essential.

SafeObservableSubscription was implemented way before SingleAssignmentSubscription. I'd support the latter one and drop the former.

[headinthebox]

I like MultipleAssignmentSubscription, SerialSubscription, SingleAssignmentSubscription hierarchy.

[akarnokd]

Why not:

• AbstractAssignmentSubscription
  • MultipleAssignmentSubscription
  • SerialSubscription
  • SingleAssignmentSubscription

/ adds setSubscription(...) throws if sets twice (like "val")
SingleAssignmentSubscription extends BooleanSubscription

// allows to setSubscription twice, leaves previous unchanged (like "var")
MultipleAssignmentSubscription extends SingleAssignmentSubscription

// allows to setSubscription twice, unsubscribes old one.
SerialSubscription extends MultipleAssignmentSubscription previous

Here I'd need to suppress/override onPreSwap in MultipleAssignmentSubscription instead of overriding an empty onPreSwap with the state check.

[benjchristensen]

Inspired changes done in #661