Check explicitly for Session ID naming collisions

[LswaN58]

We've always operated from assumption that Session IDs never collide across players. We have no reason to expect they do, with one clear exception: most games with Player IDs have some menu-level interactions available before the Player ID actually gets assigned. Thus, we end up with some events within the session having a valid Player ID, and some with null.

Down the line, this ends up looking like multiple sessions with same ID; one with a player ID, one attributed to the null-player.

Thus, we should start checking for naming collisions explicitly, where collisions between two valid Player IDs results in a severe warning message, and collisions between null and a valid Player ID results in backfilling the Player ID into the null events, as described in the subissue.