extend ddns statistics

We should consider adding new statistics to kea ddns that would track how many of queue overflows kea ddns encountered and/or how many updates were dropped from queue due to it's overflow (error log message DHCP_DDNS_QUEUE_MGR_QUEUE_FULL application request queue has reached maximum number of entries 1024)

I've run some tests on in the perf lab, ddns works stable but there are plenty of those error messages in the logs.