Fix OverflowException in MultiSubscriptionSubscriber (#2576)
This commit fixes an OverflowException when using concatMap and/or
Flux.just, finding its root in `MultiSubscriptionSubscriber` request
accounting.
It changes the underlying subscription of `Flux.just` to become a
ScalarSubscription, as the lack of volatile was causing race conditions.
It also allows to set up a predefined `stepName` for ScalarSubscription,
in order to maintain the stepName of `Flux.just` as it was before.
java.lang.AssertionError: java.lang.AssertionError:
Expecting:
<SYNC>
and actual:
<UNKNOWN>
to refer to the same object
java.lang.AssertionError:
Expecting:
(93 more lines...)
