BaseProcessor back-pressure fix backport from #1282

[danielkec]

There is a problem with BaseProcessor request count:
For example if map operator is requested like this:

request(2);
request(2);
request(2);

his publisher will receive:

request(2);
request(4);
request(6);

That can lead to flooding subscriber if proper back-pressure is applied

Signed-off-by: Daniel Kec daniel.kec@oracle.com

[olotenko]

I've seen this, but did not raise an issue, because it is not apparent if it can be triggered outside a compliance test. Eg if all known uses of BaseProcessor ensure subscription is available before the downstream subscribes, the issue with request counter does not manifest itself.
beebd46#diff-ca92013a99c748b99a9e08660b9dea17R108 - just don't call downstream onSubscribe until you have received onSubscribe from upstream. Then you don't need any of the machinery to track requests that have arrived before there was anyone to drain the requests to.

[danielkec]

@romain-grecourt Can I ask for review pls, it seems to be severe bug enough to be backported