Notifcation configs may have multiple notification destinations.
This commit changes the pipeline so that each one has its own
retry and deduplication logic.
Retrying notifier is added to the end of the pipeline where it retries
sending out the final notifications until the context times out.
Exponential backoff is used.
Setting and getting of context values are done via helper
methods in the notifier package. The used keys are unexported. Thus,
we ensure that external code cannot overwrite the values and the type
is always correct.
