Draft: map-agent: serialize beacon requests

This issue reproduces on eagle-eagle setup! My tests show, that hostapd is unable to handle few req_beacon calls in a row if there's virtually no gap between each consecutive request. At least some minimum delay between consecutive request is required - adding sleep(1) solves this issue. But delay is not a proper fix.

I'm trying to figure out now on how to best serialize consecutive ubus calls (request_neighbor) towards wifimngr (which will end up in hostapd: req_beacon).