Add BackOffExecution to isolate state

 This commit separates the BackOff configuration from an actual
 execution. BackOffExecution now contains all the state of a
 particular execution and BackOff is only meant to start (i.e.
 create) a new execution.

 The method "reset" has been removed as its no longer necessary:
 when an execution does not need to be used for a given operation
 anymore it can be simply discarded.

 Issue: SPR-11746