LoadingMessageSourceProvider: add a comment about another potential bug

This one is quite unlikely, however it is real. Since we call .setExpiry() in
the constructor, the first time the scheduled thread runs, it may find that the
sources map is null!

The fix is probably to add an AtomicBoolean that will be read in
.getMessageSource(), if it is false then set it to true and run .setupExpiry()
from there.