I have an idea how to implement mixed protocol support for a2s_info.

Split requests flow to two buckets. Fill buckets sequentially.
First bucket for common (no challenge) requests and limited.
Second is for new (with challange) requests and no limited.

When first bucked filled (rate limited exceed) all next responses will be generated with challenge request.

Profits:
Legit clients with updated Steam will see server all time
DDoS affect only old clients, who still not support new protocol
We do not spent a lot of resources to answer to each client
We do not spent a lot of time to create very-very-very smart system to filter spoofed IPs

Related to #87