Scoreboard teams are incompatible with username changes

When a player changes their Minecraft username, any scoreboard teams they are a member of are not updated to their new username. Looking at scoreboard.dat with an external NBT viewer, I can see that UUIDs are not stored with players on teams.

This issue can be reproduced by:

1. Create a scoreboard team, and add a player to the team.

2. Change the username of the player, and the player will no longer be a part of the scoreboard team they were added to.

I have tested this on a vanilla Minecraft 1.8.4 server. It's a bit of a problem when you store players on scoreboard teams for the long run, like I do.

The issue is likely present on all versions of 1.8, but I have not tested those yet.