Add a flush operation to make sure all previous datapath operations are flushed from the shm queue

[crazyboycjr]

Say the user write this code id.post_recv(...); id.accept();

Since the post_recv is an asynchronous data path operation, it could happen after the accept has happened.

Another way to prevent this is to direct all control path operations into a datapath notification in the datapath shared memory queue. This ways does not require changes in user code.

[crazyboycjr]

It is always preferred to flush all existing datapath operations before executing a control path operation.