Updater update: expand SettingsStore types to include DateTime and TimeSpan, push storage-specific conversions down into the relevant SettingsStore, add TTL to updater settings and add LastCheck/NextCheck/Update cached state.
git-svn-id: http://svn.uktrainsim.com/svn/openrails/trunk@2119 a9f3cfff-6c41-7446-b0ca-052bd761e6eb