As the storage system changed drastically in v3.0 (UUIDs, per-player file separation, removing old unused fields, etc.), the maps created with v2.7 (or earlier) can not be retreived at the moment.

This migration system will load the old map's data, retreive the player's UUIDs from Mojang, backup everything (old and new map data), load and merge them (maps created after switching to v3.0 but before the migration won't be lost).

Implemented in revision 2a49d94.