main/stasis_endpoints: Fix ContactStatusChange JSON for roundtrip_usec field

The JSON packing for the ContactStatusChange event forgot to include the
roundtrip_usec field. As a result, the field never showed up in any event,
even when the data was available. This patch corrects that error by properly
packing the JSON blob with the data.

Change-Id: I8df80da659a44010afbd48f645967518ff5daa17