Save avg. meeting size to improve LB

[samuelwei]

Is your feature request related to a problem? Please describe. Moodle sends the size of course participants with the bbb-meeting-size-hint param to the create call. This can be used by LB to estimate the load of a meeting during the start phase.

Describe the solution you'd like Calc. the avg. of the max. participants of a room based on the last x meetings. If none, skip. Use this metric prefered for choosing the server with the lowest usage. Also pass this parameter to the BBB server, in case that is a LB, so it can be used to select the best server.