Improve cursor close and connection close behaviour #529

From #405 (comment)

In my head the logic should be:

• On cursor.close()
  • Cancel any already executing queries if they exist
  • Perform implicit ROLLBACK if within a transaction
  • Set some boolean so that calling any method of the cursor leads to Error

We don't do steps 2 and 3 here (both required by DB-API).

• On cursor.cancel() - this is not a DB-API specified method so the behaviour is up to us to specify
  • Cancel any already executing queries
    • If query already finished, return False
    • If query is running and was cancelled successfully, return True
    • If no query was executed at all (how to differentiate between this and already finished? - presence of query on the cursor?) then raise Exception