Rework map session

This commit restructures the map session in to a struct
holding the state of what is needed during its lifetime.

For streaming sessions, the event loop is structured a
bit differently not hammering the clients with updates
but rather batching them over a short, configurable time
which should significantly improve cpu usage, and potentially
flakyness.

The use of Patch updates has been dialed back a little as
it does not look like its a 100% ready for prime time. Nodes
are now updated with full changes, except for a few things
like online status.

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

hscontrol/db/preauth_keys.go

@@ -92,10 +92,6 @@ func CreatePreAuthKey(
 		}
 	}
 
-	if err != nil {
-		return nil, err
-	}
-
 	return &key, nil
 }