Update Player SQL Indexing to SteamID64 from UniqueID

According to the Garry's Mod Wiki, Player:UniqueID() is unsafe and is capable of colliding. Player:SteamID64() is guaranteed not to collide, and is a better choice.

This should be done in an update that's liable to nuke the SQL database anyway (like Mining Rewrite), as this will do so unless we go about converting everything manually.