1.x: NotificationLite - reduce allocations

[davidmoten]

A lot of commonly used operators allocated an instance of NotificationLite as a field in a subscriber but for no great type-safety benefit. I've cut these fields by converting all instance methods of NotificationLite to static methods and removing the NotificationLite.instance() method.

The changes are trivial till you get to the Subjects that would pass instances of NotificationLite in methods of the SubjectSubscriptionManager. Still simple changes but reviewing these is a good idea.

RxJava2 goes a bit further in the internals of NotificationLite. I'd like to leave changes to the internals beyond making methods static for another PR if appropriate.

[codecov-io]

Current coverage is 84.66% (diff: 78.12%)

Merging #4621 into 1.x will increase coverage by 0.15%

@@                1.x      #4621   diff @@
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  Files           274        274          
  Lines         17807      17766    -41   
  Methods           0          0          
  Messages          0          0          
  Branches       2727       2727          
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- Hits          15049      15042     -7   
+ Misses         1887       1860    -27   
+ Partials        871        864     -7   

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