Make notifyEvent asynchronous

[rcannood]

Makes the notifyEvent functionality asynchronous, as requested by @pditommaso in #6443:

This seems going into the right direction. My only connect is about the notification events, here

https://github.com/nextflow-io/nextflow/pull/6443/files#diff-cb4252ec3616353913180e8e6eb95c249b4d591daf4143283e543be188568685R105-R111

Likely those events should be emitter async in this context for two reasons:

1. Make sure the target path is accessible when the event is received

2. Avoid that core runtime performance could be impacted by compute/data intensive that could be implemented by third-parties plugins

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Update: I had to change the return type of notifyEvent so notifyFlowComplete and notifyError can wait until all futures are resolved before continuing.

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