2.x: add sample() overload that can emit the very last buffered item

[akarnokd]

The sample() operator in 1.x has been changed to always emit the very last buffered item before completion for 1.1.3: Discussion, PR.

This change has been forgotten in 2.x (because 2.x sample() was implemented several months earlier) but unlike 1.x, I don't want to break existing use cases, hence the introduction of 6 overloads (3 for each base type) that let's one define the "tail" behavior.

Note that the associated marble diagrams are inconsistent with the operators implemented, we need a new diagram where the timed-sample also emits the last value:

and one new diagram where the sampler-sample doesn't emit the last value:

The default sampler-sample should use this and the emitLast == true version can use the current diagram.

(I don't have a Mac thus can't run OmniGraffle).

Related: #4952

[codecov-io]

Current coverage is 95.57% (diff: 98.54%)

Merging #4955 into 2.x will decrease coverage by 0.15%

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