This commit optimizes Flux <-> Mono conversions in our codebase by
avoiding to hide that conversion from Reactor.

This tries to keep conversions sequentially so that they can be detected
by Reactor and optimized. In Spring WebFlux, this means keeping the
conversions at the edges of a method implementation (right when getting
an input parameter, and before returning it as a result). If those
conversions are made between other operators, Reactor might not be able
to detect those conversions and optimize them.

Issue: SPR-17203