pf: cope with SCTP port re-use

Some SCTP implementations will abort connections and then later re-use the same
port numbers (i.e. both src and dst) for a new connection, before pf has fully
purged the old connection.

Apply the same hack we already have for similarly misbehaving TCP
implementations and forcibly remove the old state so we can create a new one.

MFC after: 2 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")