Improve interface of Inputs in Filebeat

[ph]

Input are responsable of taking the inputsource (UDP, TCP, Log) and create the necessary logic and plumbing to get that data out.

Inputs are either persistent like the UDP/TCP or periodically called like the Log inputs, the current interface that all inputs implements is the following one.

type Input interface {
	Run()
	Stop()
	Wait()
}

We can do a few minor improvements:

• Allow Run()|Stop()|Wait() to return an error, useful in persistent inputs.
• Maybe Allow inputs to configure by code the frequency of the polling instead of having this done externally.

[botelastic]

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[botelastic]

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