* Add Reactive mode for AbstractPollingEndpoint
* When `SourcePollingChannelAdapter.outputChannel` is a
`ReactiveStreamsSubscribableChannel`, use `Flux.generate()` for polling
* Refactor `AbstractPollingEndpoint` to remove redundant `Poller` class
in favor of lambda
* Extract `pollForMessage()` method to handle TX states instead of
`Poller` class previously
* * Rebase and fix conflicts
* Polishing for GatewayProxyFactoryBeanTests
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