LiveUpdate db updates working

I've tested the behavior of the thread that waits for the LiveUpdate to
be finished, and it does get signaled and exit cleanly when the
LiveUpdate is GCed instead.

Made finishedLiveUpdate wait for the thread to finish updating the
database.

There is a case where GC doesn't happen in time and the database is left
with a live update recorded in it. This should not be a problem as such
stale data can also happen when interrupted and will need to be detected
when loading the database.

Balanced preferred content expressions now call startLiveUpdate.

Types/RepoSize.hs

@@ -27,19 +27,11 @@ newtype MaxSize = MaxSize { fromMaxSize :: Integer }
 -- Used when an action is in progress that will change the current size of
 -- a repository.
 --
--- The live update has been recorded as starting, and filling the MVar with
--- the correct UUID, Key, and SizeChange will record the live update
--- as complete. The Bool should be True when the action successfully
--- added/removed the key from the repository.
---
--- If the MVar gets garbage collected before it is filled, the live update
--- will be removed.
---
 -- This allows other concurrent changes to the same repository take
 -- the changes to its size into account. If NoLiveUpdate is used, it
 -- prevents that.
 data LiveUpdate
-	= LiveUpdate (MVar ()) (MVar (Bool, UUID, Key, SizeChange))
+	= LiveUpdate (MVar ()) (MVar (UUID, Key, SizeChange)) (MVar ())
 	| NoLiveUpdate
 
 data SizeChange = AddingKey | RemovingKey