Add basic realm limits

[baranga]

Add basic realm limits:

• Maximum number of online clients
• Maximum message size (broker docs)

Not covered:

• Maximum number of messages per session per second (broker only supports setting a global session limit but not different limit for each session; this needs to be implemented manually; upper hard cap could be achieved by using throttle modifier)
• Maximum session lifetime (broker docs; this seems to be not supported by the current broker version we use)