map-agent: add TODO to split channel_report in bcn_request

Currently clients will likely not handle the request properly if channel_report is used in beacon metrics query. I.e. one can not send one request to cover all opclasses/channels.

• Instead the report must be broken down into separate queries for each oplcass / channel pair individually.
• This should be done in agent.
• Another thing is that one must not send the requests in a row without breaks, as hostapd will not be able to handle it properly. => One needs to enqueue the requests somehow - e.g. keeping the requests in agent {} struct and using uloop timer to send them sequentially keeping some minimal delay between the requests.