2.x: cleanup based on IntelliJ 2017.1 inspections

[akarnokd]

This PR is the result of running IntelliJ inspections on the code:

• replace unsubscribe with cancel and dispose in the main classes to better match the 2.x terminology
• write out a couple of local variables
• remove unnecessary return and continue statements
• remove unnecessary variable writes
• remove unnecessary and unused fields
• add final to some fields
• fix typos

[codecov]

Codecov Report

Merging #5222 into 2.x will increase coverage by 0.12%.
The diff coverage is 97.29%.

@@             Coverage Diff              @@
##                2.x    #5222      +/-   ##
============================================
+ Coverage     96.01%   96.13%   +0.12%     
+ Complexity     5747     5743       -4     
============================================
  Files           628      628              
  Lines         41085    41060      -25     
  Branches       5698     5692       -6     
============================================
+ Hits          39446    39473      +27     
+ Misses          657      629      -28     
+ Partials        982      958      -24

Impacted Files	Coverage Δ	Complexity Δ
...a/io/reactivex/internal/util/MergerBiFunction.java	92.3% <ø> (ø)	12 <0> (ø)	⬇️
...operators/observable/ObservableConcatMapEager.java	99.45% <ø> (-0.01%)	2 <0> (ø)
...vex/internal/operators/flowable/FlowableCache.java	92.61% <ø> (-1.39%)	7 <0> (ø)
...ors/observable/ObservableDistinctUntilChanged.java	100% <ø> (ø)	2 <0> (ø)	⬇️
src/main/java/io/reactivex/Flowable.java	100% <ø> (ø)	522 <0> (ø)	⬇️
...al/operators/flowable/FlowableFlattenIterable.java	97.22% <ø> (-0.02%)	4 <0> (ø)
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	506 <0> (ø)	⬇️
src/main/java/io/reactivex/Single.java	99.32% <ø> (ø)	131 <0> (ø)	⬇️
...ex/internal/operators/flowable/FlowableWindow.java	90.67% <ø> (+5.07%)	4 <0> (ø)	⬇️
src/main/java/io/reactivex/Scheduler.java	98.83% <ø> (ø)	11 <0> (ø)	⬇️
... and 83 more

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[vanniktech]

You might want to leave it as it was before since that way you won't allocate an iterator

[akarnokd]

Hmm, yeah, I forget that who knows what optimization the Android VM does (the JVM's escape analysis optimizes this nicely). I'll restore the original indexed version.