1.x: fix Completable.concat to use replace (don't dispose old)

[akarnokd]

Fixes the same bug as with the 2.x Completable.andThen and Completable.concat.

See #5694 & #5695.

[codecov]

Codecov Report

Merging #5696 into 1.x will decrease coverage by 0.05%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                1.x    #5696      +/-   ##
============================================
- Coverage     84.24%   84.19%   -0.06%     
+ Complexity     2888     2886       -2     
============================================
  Files           290      290              
  Lines         18256    18256              
  Branches       2495     2495              
============================================
- Hits          15380    15370      -10     
- Misses         1992     2002      +10     
  Partials        884      884

Impacted Files	Coverage Δ	Complexity Δ
...l/operators/CompletableOnSubscribeConcatArray.java	87.5% <100%> (ø)	2 <0> (ø)	⬇️
...perators/CompletableOnSubscribeConcatIterable.java	92.59% <100%> (ø)	3 <0> (ø)	⬇️
.../java/rx/internal/util/unsafe/MpscLinkedQueue.java	72.72% <0%> (-18.19%)	8% <0%> (-3%)
...rx/internal/util/atomic/MpscLinkedAtomicQueue.java	74.19% <0%> (-12.91%)	7% <0%> (-1%)
...java/rx/internal/schedulers/ExecutorScheduler.java	81.69% <0%> (-2.82%)	2% <0%> (ø)
...n/java/rx/subscriptions/CompositeSubscription.java	75.32% <0%> (-1.3%)	25% <0%> (ø)
...n/java/rx/subjects/SubjectSubscriptionManager.java	80% <0%> (-0.72%)	15% <0%> (ø)
...in/java/rx/internal/operators/OperatorGroupBy.java	90.81% <0%> (-0.71%)	5% <0%> (ø)
...main/java/rx/internal/operators/OperatorMerge.java	85.32% <0%> (-0.46%)	7% <0%> (ø)
src/main/java/rx/Completable.java	83.41% <0%> (+0.12%)	103% <0%> (+1%)	⬆️
... and 4 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 396b610...110e343. Read the comment docs.