Run handler on STM retry

It would be nice to have a function like:

-- | Perform a series of STM actions atomically.
--
--   Whenever the transaction retries, run the handler. If it doesn't return 'Nothing',
--   stop the retrying and return the value given by the handler.
atomicallyWithRetryHandler :: STM a -> IO (Maybe a) -> IO a

This would allow tracking retry statistics, printing debug messages on each retry, etc. without resorting to unsafeIOToSTM which can't safely do things like printing.

Trac metadata

Trac field	Value
Version	7.10.2
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries (other)
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