Support Cursor.rowcount and close finished queries

[csringhofer]

With current Impala server rowcount support needs DMLs to be closed with CloseImpalaOperation() as there is no simpler way to get the number of modifed rows. See https://issues.apache.org/jira/browse/IMPALA-12647 for alternatives.

This change adds option close_finished_queries for cursors with default True. Setting it to False brings back the old behavior.

If queries are closed after finishing queries, calling get_log RPC is no longer possible. If close_finished_queries is true then the logs are fetched and stored before closing to query to be able to return the saved results with get_log. Generally get_log shouldn't be a too expensive RPC.

Another potential side-effect is that get_profile may fail as Impala can discard the runtime profile after the query is closed (see Impala flag query_log_size).

Despite the above side effects closing the queries seems a better default behavior as it helps avoiding queries hanging in the "waiting to be closed" state and provides reliable rowcount. This is also consistent with the way impala-shell works.

Testing:

• rowcount already had good coverage in DBAPI2 compliance tests (e.g. test_mixedfetch)
• new tests were added for some missing rowcount cases and for getting warning/error log for closed queries

[joemcdonnell]

I'm not sure what our process is, should we file an issue on github for this?

[csringhofer]

I'm not sure what our process is, should we file an issue on github for this?

There is already an issue about rowcount: https://github.com/cloudera/impyla/issues/302 I am also unsure about the process.