2.x: fix publish(Function) not replenishing its queue

[akarnokd]

I was working on converting the Monte Carlo PI benchmark to RxJava 2 and it turned out I forgot to replenish the internal queue of publish(Function) in case the upstream was not sync-fuseable.

This PR fixes the replenishment problem for non-fused and async-fused sources by requesting after 75% consumption.

In addition, if an inner subscriber cancelled while it was at request 0, the other inner subscribers might not have resumed draining.

I've also fixed two unit tests with wrong timeout unit of measure that randomly failed the tests on a slow machine or travis (#4902).

[codecov-io]

Current coverage is 95.50% (diff: 100%)

Merging #4943 into 2.x will decrease coverage by 0.11%

@@                2.x      #4943   diff @@
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