Cleanup and improve SlidingWindow decorator.

Now caches mlangs like COUNT properly.
Is properly re-entrant ( so repeated queries don't go to the network if a network request is pending ).
Fix context reference in SplitDAO
More sliding window DAO bug fixes.
Only refresh on 401.
Add a SimpleScrollView alternative to ScrollView.
Add skip to NullDAO.
SyncManager: Enable the startSync action even if a sync request is in progress.

This way if you force a sync you can immediately enable sync whlie the forced sync request is pending
Remove unneeded rerence to issueCommentDAO() since its provided via the context now.
Merge branch 'master' of github.com:foam-framework/foam