Changes

sctp_proxy: use a record for storing state data

Maps are convenient, but they are also more permissive: field names
are unchecked at compile time, making it easier for typos or unexpected
fields to slip through.  Records, on the other hand, are stricter and
more declarative.  Their structure is explicitly defined, the set of
fields is fixed, and the compiler can validate field names.

Change-Id: Ice9c255c0cf14db0a216bb078198b9b9c76d22a7