RE: Perform streaming logical transactions by background workers and parallel apply

Hi, 

while reading the code, I noticed that in pa_send_data() we set wait event
to WAIT_EVENT_LOGICAL_PARALLEL_APPLY_STATE_CHANGE while sending the
message to the queue. Because this state is used in multiple places, user might
not be able to distinguish what they are waiting for. So It seems we'd better
to use WAIT_EVENT_MQ_SEND here which will be eaier to distinguish and
understand. Here is a tiny patch for that.

Best Regards,
Hou zj