Deprecate and rename two timer overloads to interval

[abersnaze]

The existing six methods below didn't make sense and caused some confusion.

timer(delay, timeUnit[, scheduler]);  --> emits 0 after delay and completes
timer(delay, period, timeUnit[, scheduler]); --> emits 0 after delay and then i++ after every period forever
interval(period, timeUnit[, schduler); --> emits i++ after every period forever

I felt that the middle method acted more like the third method interval than first method timer. This PR is to make this change.

 timer(delay, timeUnit[, scheduler]);  --> emits 0 after delay and completes
+@Deprecated
 timer(delay, period, timeUnit[, scheduler]); --> emits 0 after delay and then i++ after every period forever
+interval(delay, period, timeUnit[, scheduler]); --> emits 0 after delay and then i++ after every period forever
 interval(period, timeUnit[, schduler); --> emits i++ after every period forever

PS: yes, I understand that we can't delete the deprecated timer method.
@davgross and if this PR is merged will have to change the images.

[DavidMGross]

I'd recommend leaving the documentation in place even when deprecating the method.

[abersnaze]

Okay, I think I'm done changing the PR unless there are other comments.

[akarnokd]

I'm not in favor of these changes.

[abersnaze]

any particular reason?

[akarnokd]

If it were only a matter of aliasing, I wouldn't see any problem, but you deprecate a historical method just because the name. Every use place need to be changed to avoid deprecation warnings in RxJava and outside it.

[abersnaze]

I think it improved clarity of API and is directly related to a bug we had here. This PR already takes care of migrating the RxJava usages of timer to interval.

[akarnokd]

The timer-interval confusion apparently is quite common so I'm going to merge this. Thanks for the changes.