privilege: store roster cache in database: - rosters are now stored on database and restored on startup. This way, presence map can be restored without the need to wait for all contact to send presence again - roster version are checked, if a new version is received, presence map is updated accordingly - roster are not retrieved if presence are received in a too short delay (see ROSTER_TTL), to avoid using too much resources if a client connect/disconnect a lot The current behaviour works around XEP-0356 limitations. An update of the XEP will be needed to get roster pushes and roster version.