Previously, a save could possibly be loaded twice given enough latency
discrepancy between servers. The server only verified that it deleted
some save with the given password, not the save it is trying to
load. This is fixed by also checking the SaveID column that is random
and globally unique (except for the old NULLs). Since users can't create
new saves with NULL SaveID, these pose no problem.
Also change the default UUID for saves without save ID to something
nonzero, so we ca...