pf: support SCTP-specific timeouts

Allow SCTP state timeouts to be configured independently from TCP state
timeouts.

Reviewed by: tuexen
MFC after: 1 week
Sponsored by: Orange Business Services
Differential Revision: https://reviews.freebsd.org/D42393

(cherry picked from commit ca9dbde88122beb079b55fb4580b200f73044da6)