We now use Flux.usingWhen() instead materialize/dematerialize operators
to reuse Reactor's resource closure.
Until usingWhen() accepts a BiFunction to consume error signals,
we need to map error signals outside of usingWhen which requires
re-wrapping of the ReactiveTransaction object.
Also, reuse the current TransactionContext to leave Transaction
creation/propagation entirely to ReactiveTransactionManager instead
of creating new TransactionContexts.
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