README.async

async operation support in bundle
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- one outstanding operation is supported
- only gentable operation support the async operation (not for flow table)

OF ConnectionManager:
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- bundle task will yield if there is an outstanding operation
- if the operation returns INDIGO_ERROR_PENDING, the bundle task will mark
  that operation as pending, and resume the task from that operation.

OF StateManager:
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- The gentable operations provide new getnable operation methods which
  gentable drivers should implement them if those operations are going to
  be asynchrous. (Look for add4(), modify4(), and del4() descriptions in
  indigo/of_state_manager.h)
- The OF StateManager only expects the INDIGO_ERROR_PENDING returned from the
  the new methods (add4(), modify4(), and del4()).
- If INDIGO_ERROR_PENDING is returned, the OF StateManager will mark the
  outstanding operation and return the same status to OF ConnectionManager.
  The OF StateManager will put the remaining work of that operation on hold.
- A resume function, indigo_core_gentable_entry_resume(),  must be called
  by the driver when the operation is completed (either successful or error).
- The StateManager will perform the remaining work for that operation. If
  the final status is successful, the entry will be added or deleted in the
  gentable state tracked by OFStateManager.
- The StateManager will release that operation from pending state so that
  OF ConnectionManager can resume the next operation.

Gentable Driver:
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- The driver must implement the new *4 methods if it wants to support async
  operations.
- Return an INDIGO_ERROR_PENDING to OF StateManager if the operation is
  asynchronous. 
- Call the indigo_core_gentable_entry_resume() when the operation is completed.
- There is a "op_ctx" in the *4 methods. This op_ctx is the used by the
  OF StateManager in its resume function.