The OPAL firmware only supports a finite number of in-flight asynchronous
operations. Rather than have each subsystem try to manage its own, use a
central management service to hand out tokens.
More work can be done to improve asynchronous behavior, such as funneling
things through a future OPAL heartbeat handler, but capabilities will be
added as needed.
Augment the existing consumers (i2c and sensors) to use this new API.
MFC after: 4 weeks
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