Async ajax calls for some visualisations

If the dashboard contains lots of widgets/visualisations they block
eachother (javascript thread blocks) by doing lots of sync calls (page
load with 4-5 widgets could be 2-3sec). This change makes the rawdata
and realtime widget to use async ajax instead with callback which
doesn't block the javascript thread in browser - this makes the
dashboard feel alot snappier and page load is about 0.5sec instead in
comparsion on my test. Also the widgets does not "hang" during zooming
and time range change since the data fetching is not syncronous anymore.