refactor connection limits and throttling

It's not really used and I'd rather not have it here unless I'm able to actively maintain it properly and build out the web interface.

I might re-add it later but for now I'd rather not have it unless anyone's actively using it.

also, remove the self-notification check; clients are probably not
monitoring themselves, and it doesn't much matter if they are

fix various data races, including 2 introduced by #139

* Lower ident timeout to 1.5 seconds, as discussed in #oragono
* Enforce connection limits at the granularity of ipv4 /32's and ipv6 /64's
* Raise the limit on number of new connections to 64 every 10 minutes
  (for the benefit of people behind large IPv4 NATs)

refactor [dk]linemanagers

* Synchronize map accesses
* Be more aggressive about cleaning up expired lines
* CIDR dlines did not work (nil dereference)