Improved agent operation recovery process.
Improved agent operation recovery process.

This patch makes the agent walk the operation status update streams
directories in order to generate the list of streams to recover, instead
of generating it from the checkpointed `ResourceState` message.

This prevents the agent from asking the operation status update manager
to recover streams that haven't been created yet.

The patch also makes the agent garbage collect operation status update
streams if no correspondng operation is present in the checkpointed
state. This can happen after the agent fails over while processing the
acknowledgement of a terminal operation status update.

Review: https://reviews.apache.org/r/69977/

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