Set a default lastActivityTime on Conference creation
Set a default lastActivityTime on Conference creation

Summary:
Conferences have a default lastActivityTime of zero.
It could be possible for VideobridgeExpireThread
to run immediately after a conference is created,
thereby expiring the conference.

gpolitis/jitsi-videobridge,jitsi/jitsi-videobridge,jitsi/jitsi-videobridge,jitsi/jitsi-videobridge,jitsi/jitsi-videobridge,jitsi/jitsi-videobridge,jitsi/jitsi-videobridge,jitsi/jitsi-videobridge,gpolitis/jitsi-videobridge,gpolitis/jitsi-videobridge