2.x: fix Observable.repeatWhen & retryWhen not disposing the inner

[akarnokd]

The Observer that is subscribing to the Observable returned by the handler function was not properly linked to the disposing of the end Observer. This PR fixes it by making that Observer do async Disposable management and make sure the end Observer receives a disposable list with both the main and the handler Disposables.

The Flowable versions worked properly but added unit tests to them regardless.

Reported in #4782.

[codecov-io]

Current coverage is 95.72% (diff: 91.66%)

Merging #4783 into 2.x will decrease coverage by 0.14%

@@                2.x      #4783   diff @@
==========================================
  Files           570        570          
  Lines         36713      36717     +4   
  Methods           0          0          
  Messages          0          0          
  Branches       5555       5554     -1   
==========================================
- Hits          35200      35149    -51   
- Misses          617        646    +29   
- Partials        896        922    +26   

Powered by Codecov. Last update 3300d19...02aa31c