trap-manager: Prevent deadlock when installing trap policies

trap-manager: Prevent deadlock when installing trap policies

Because the write lock was held while calling add_policies() on
child_sa_t, which finishes with a call to child_state_change() on bus_t,
a deadlock would ensue if CHILD_SAs are concurrently being established,
which also causes a call to child_state_change() that will require
the read lock in trap_manager_t.

No locks are now being held while creating the CHILD_SA and installing the
trap policies.