DfsObjectDatabase: Expose PackList and move markDirty there

DfsObjectDatabase: Expose PackList and move markDirty there

What's invalidated when an object database is "dirty" is not the whole
database, but rather a specific list of packs. If there is a race
between getting the pack list and setting the volatile dirty flag
where the packs are rescanned, we don't need to mark the new pack list
as dirty.

This is a fine point that only really applies if the decision of
whether or not to mark dirty actually requires introspecting the pack
list (say, its timestamps). The general operation of "take whatever
is the current pack list and mark it dirty" may still be inherently
racy, but the cost is not so high.

Change-Id: I159e9154bd8b2d348b4e383627a503e85462dcc6