Add an API to return load on each server i.e. number of local sessions.

[danesh-d]

The main intention of this patch was to make it possible to get the local load of each instance of Openfire in a clustered environment so the load balancer would know where to route the traffic.

There might be better solutions but this is our way to solve the problem so far. Hope this patch is useful and will be accepted.

[Redor]

Sry for a super late response. Thy for the PR.
I think it is a better way, to create an own "Entity" class instead of using SessionEntity for that.
Otherwise we have the "load" parameter also by getting the Session Information.
And maybe we should consider to use another name, like "clients", instead of load.