Client exponential backoff about go-micro HOT 3 CLOSED

Thinking about this further and discussing with @boyand about context.{Cancellation, Deadline}, etc. It's clear that there's a variable number of ways in which people will want to execute call strategies.

So firstly, rather than just having a constant as the seed for backoff option, it probably makes sense to support a backoff function, this allows the user to define backoff based on external components if need be.

Secondly, it may make sense to support a CallFunc as a client Option or CallOption which allows you to override the default behaviour of the call. A call usually entails, selecting a node using the selector, dialing using the transport and then trying to execute the call until success, retry limit or timeout is hit. CallFunc would allow all of this functionality to be swapped out and a user could define the cancellation behaviour.

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Backoff function. #70

Will address CallFunc separately.

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Merged

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