If there is an asynchronous query, polling a cursor that did not
initiate it will raise an exception. Polling while there is no
asynchronous query underway still works, because the user needs to
have a way to get asynchronous NOTIFYs.
When a large query is sent to the backend (and probably in high
concurrency situations), writing the query could block. In
this case PQflush() should be called until it returns 0. The test checks
this is done correctly.
Some methods were forbidden in asynchronous mode, the isolation level
of an asynchronous connection is not always 0 and these changes
influenced expected test results.
