lxd/daemon: Call cluster.EventsUpdateListeners asynchronously from startClusterTasks
lxd/daemon: Call cluster.EventsUpdateListeners asynchronously from startClusterTasks

This way when a clustered LXD server starts, creates a cluster or joins a cluster, the current initial set of event listeners are connected.

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

lxc/lxd,stgraber/lxd,stgraber/lxd,stgraber/lxd,hallyn/lxd,lxc/lxd,lxc/lxd,lxc/lxd,stgraber/lxd,hallyn/lxd,hallyn/lxd,hallyn/lxd,stgraber/lxd,stgraber/lxd,stgraber/lxd,lxc/lxd,lxc/lxd,lxc/lxd,hallyn/lxd,hallyn/lxd