releaseSubscriptions

Drops whatever this session subscribed to, if anything.

Called when a session closes over a connection that will outlive it: leaving the registrations in place would hand the next borrower of a pooled connection someone else's subscriptions, and let them pile up over the connection's lifetime. Sessions that never called listen pay nothing.

Sent as a statement about the connection rather than as the session's own work, so that it never opens the transaction a BEGIN may be waiting for: inside one, UNLISTEN takes effect only at its commit. Where the session leaves open a transaction its connection's owner began, it goes out inside that one, and stands or falls with it as the session's LISTENs there do.