Fix of the canvas update timer

- If you start the update timer and there is no data in the archive for the selected variable in that period a segmentation fault happens. The timer is not yet initialized, but the stop method is called on it -> Segmentation fault.

Steps to reproduce:
1) Configure local database for testing (from the scripts you provided).
2) Run webapp.
3) Configure start time period to: 2013-08-29
4) Add m3sr/vacuum/ip-ss81/pressure to the graph.
5) Select "1 second" continually update period.

- When starting the timer (updateCanvasContinuallyStart) a new instance is always created, but when stopping the timer(updateCanvasContinuallyStop) the instance is not deleted -> Memory leak.