Inaccurate timeline for "Rows Available"

While debugging IMPALA-6932, it was noticed that the "Rows Available" metric in the query profile was a short duration (~ 1 second) for a long running limit 1 query (~ 1 hour).

Currently, it tracks when Open() from the top-most node in the plan returns, not when the first row is actually produced. This can be misleading. A better timeline would be to return true when the first non-empty batch was added to the PlanRootSink. 

We should consider changing the definition of the FINISHED state accordingly as well, so that we don't transition to FINISHED until a row is actually available to fetch immediately.