Proposed cancel changes.

[rweikusat]

Splits pg_db_cancel into a sychronous cancel operation callled pg_db_cancel and a function which just sends a cancel request to the server without waiting for the result called pg_db_send_cancel. Fixes the cancel result handling code in pg_db_cancel by invoking pg_db_result to wait for the result of the possibly cancelled query instead of partially duplicating the result handling code in pg_db_cancel itself.

Introduces a new internal function called do_send_cancel which contains just the cancel request sending logic and calls that from all places in the original code which used to duplicate this functionality.

Changes pg_db_result such that it no longer treats cancelled queries as fatal errors but returns zero and a SQLSTATE of 57014 instead.

Modifies t/08async.t to enable AutoCommit so that cancelled queries no longer cause transaction aborts which would need to be dealt with by rollbacks. Removes a now redundant $dbh->commit(). Changes handle_old_async cancel path to only issue a rollback if actually in a transaction to avoid warnings about issueing a rollback outside of a transaction.

Unrelated fix: Uses the correct TRACE macro (TRACE_PQCANCEL) when about to call PQcancel instead of the TRACE_PQGETCANCEL which was used in this place instead.

This passes the test suite plus the two tests I added for pg_db_send_cancel.