Track slice retries

[kstaken]

It would be helpful to be able to see how many times a slice retries before it is considered a success or failure. Should probably be part of the state record for the slice.

[jsnoble]

Is this still useful in case of kafka where we can't even trace to the source documents from the slice?

[godber]

Without traceability to which specific records in the kafka case, I think the benefit of this is pretty marginal.