Merge branch 'reqid-refcount'

This fixes issues with CHILD_SAs getting reestablished concurrently.
We intend to reuse the reqid of the previous CHILD_SA, however, previously
the reqids were released and up for reassignment to any other CHILD_SA
or trap policy.  This could cause the reqid to get associated with
completely different traffic selectors, as the reestablished CHILD_SA
would eventually get the requested reqid because the traffic selectors
explicitly don't have to match (to allow narrowing for CHILD_SAs based
on trap policies).

Closes strongswan/strongswan#1855