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

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

support for PROXY was removed in 43e28e2fef. After discussion,
it's worth keeping around in case of client compatibility issues,
and until /rehash support is more mature.

Don't close and reopen listeners

This prevents a deadlock:

1. rehash() is executing on the main goroutine
2. it's trying to stop a listener goroutine
3. the listener goroutine needs to hand off a new connection to newConns
4. but the main goroutine is blocked by rehash() so it can't receive it

See #134; there was a `fatal error: concurrent map read and map write`
due to unsynchronized accesses to `Server.listeners`. Now, `listeners`
is only accessed by `NewServer` and `rehash`, so it doesn't need
synchronization.

PTR records ending in . (e.g., `google-public-dns-b.google.com.`)
were considered invalid