Statement.cancel() causes lockup

[mrotteveel]

See also https://groups.google.com/g/firebird-java/c/WDsY8fBLmyo/m/r56MBQefAAAJ; it looks like cancelling a statement too quickly after execution might have some race condition or incorrectly registers or doesn't unregister a deferred operation, causing a subsequent hang of the connection waiting for data.

There currently is no clear reproduction recipe.

If a fix is found, it will probably need to be backported to Jaybird 5 as well.