Fix possibility of concurrent status reports

The coordinator may receive multiple backend status reports concurrently for a few reasons:

• The backend believes that sending the report failed and retries when in fact the first report did reach the coordinator.
• A report is sent by the reporting thread at the same time that one is sent by Finalize().
• Multiple fragments from the same backend send reports at the same time.

This has the possibility of causing races. One such race was fixed in IMPALA-5749, but we should deal with the underlying issue of concurrent reports in a more complete way, rather than just trying to fix individual races as they become apparent.