Add support for reactor-tools

[bclozel]

After chatting with the reactor team, it seems that the Hooks.onOperatorDebug() is going away and soon to be replaced by reactor-tools support, with a few differences:

• reactor-tools is a separate dependency, adds debugging support with bytecode (vs. AOP-like with operators)
• reactor-tools is generally supported (for development and production), but might not be widely compatible with all JVMs right now

With that in mind, we should not add reactor-tools as a dependency to existing starters, as this should be a conscious choice by developers to include it.

We should, if on the classpath, enable automatically the reactor-tools agent. We need to provide a configuration property to disable it in case developers need an "escape hatch".