2.x: fixes, cleanups, renames

[akarnokd]

Changes:

• fix spelling mistakes
• more consistent parameter naming scheme based on Make operator parameter names consistent across operators. #1353 (but incomplete, RC2)
• fix bugs due to wrong variable naming
• compact logic expressions
• fix Completable.doAfterTerminate not called the right time
• onErrorReturnValue renamed to onErrorReturnItem

@DavidMGross once this PR has been merged, it would be great you submitted one that has //TODO markers on those lines that need attention. The naming scheme you suggested doesn't come intuitively to me so I may skip many of them without the markers.

[codecov-io]

Current coverage is 69.48% (diff: 73.27%)

Merging #4421 into 2.x will increase coverage by 0.03%

@@                2.x      #4421   diff @@
==========================================
  Files           454        454          
  Lines         32372      32323    -49   
  Methods           0          0          
  Messages          0          0          
  Branches       5214       5214          
==========================================
- Hits          22482      22459    -23   
+ Misses         7692       7662    -30   
- Partials       2198       2202     +4   

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[JakeWharton]

👍