Support challenged A2S_INFO requests

[asherkin]

Valve released an update to the query protocol at the end of last year that allows servers to require A2S_INFO requests to contain a challenge value (same as A2S_PLAYERS and A2S_RULES) and respond with a S2C_CHALLENGE response if the challenge is missing or invalid. Blaster needs to handle receiving a S2C_CHALLENGE response instead of S2A_INFO_SRC and re-send the request with the challenge value.

L4D2 was recently updated to require challenges by default, so we're having issues querying them now.

• Steam client release: https://steamcommunity.com/discussions/forum/14/2974028351344359625/
• Information on the changes: https://steamcommunity.com/discussions/forum/14/2989789048633291344/
• Protocol changes: https://developer.valvesoftware.com/w/index.php?title=Server_queries&type=revision&diff=238327&oldid=236379
• L4D2 update: https://store.steampowered.com/news/app/550/view/4625740823862061051 (not mentioned in changelog)