Fixing the generics for merge and lift

[abersnaze]

I've tracked down to two problems #858

The first is that lift wasn't quite right. I changed the lift argument from Operator<R, T> to Operator<? extends R, ? super T>. Unfortunately Java won't let us hide that away in the Operator class so we have to redeclare that everywhere in the debug hooks but it's still better than the alternative of having to use the full Func1<? extends Subscriber<? super R>, ? super Subscriber<? super T>> everywhere.

The second problem was in the return type of merge Observable<T>. Because of operators like merge(Observable<? extends T> t1, Observable<? extends T> t2) the only thing that can be said for the return type is that it is also Observable<? extends T> but at the moment it is returning Observable<T>.

This pull request changes all of them and leads to some silly return values like

Observable<String> a, b;
Observable<? extends String> x = merge(a, b);

[cloudbees-pull-request-builder]

RxJava-pull-requests #784 FAILURE
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[akarnokd]

We should avoid return types such as A<? extends B>.

[cloudbees-pull-request-builder]

RxJava-pull-requests #787 FAILURE
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[cloudbees-pull-request-builder]

RxJava-pull-requests #788 SUCCESS
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[abersnaze]

@akarnokd you were right the return type change wasn't necessary. I've reverted and rebased the branch to clean out all the mistakes.

[cloudbees-pull-request-builder]

RxJava-pull-requests #789 SUCCESS
This pull request looks good

[akarnokd]

I almost did a PR on this when I saw you now did the same thing as I. But was it necessary to change the debugs as well?

[abersnaze]

Yes. There is a plugin hook inside lift that the debug plugin implements and spreads from there.