SingleClient may be redundant

[james7132]

https://partner.steamgames.com/doc/api/steam_api#SteamAPI_RunCallbacks

According to Steam, RunCallbacks is actually thread safe to call from multiple threads if need be, so SingleClient may be Sync, and may be able to be merged with Client. It does note that there may be a memory leak if SteamAPI_ReleaseCurrentThreadMemory is not called periodically on every thread though, so we may need a wrapper for that.