Remove hardwired top-level backoff

[bartelink]

EventStore/EventStore#38 introduced a hardwired backoff as a way to achieve feature parity with an existing implementation, as a way of punting on where such a backoff should live.

Given the fact that the implementation work in EventStore/EventStore#42 has made it clear that a cleaner solution will be possible with Equinox.Cosmos, this PR makes the backoffs pluggable so that one can plug in a hardwired workaround for EventStore/EventStore-Client-Dotnet#312 in the absence of a more appropriate soltuion in any application context.

And, yes, I'd love for someone to clean up the policy such that the way one specifies the exception, max retries and backoffs is much cleaner and discoverable than this :(