TISTUD-6584 Importing sample apps from Dashboard or Samples gives interm...

Finally. Finally ! There is a very small timing window issue between 3 threads running in parallel -

- After a sample project is imported through git repository, a thread is handling a new git repo added event.
- We try to disconnect the sample project after it is imported.
- Git label lightweight decorator is trying to calculate the changed resources and refreshing the contents.

All those 3 threads are trying to refresh the git contents and we have a thread executor that was set to execute maximum of 3 tasks at a time. There are instances that we end up trying to execute the disposed thread executor (after sample project is disconnected) and at other instances, 1st and 3rd thread tried to over-execute more than 3 tasks at the same time.

The fix here addresses the timing issues and give little more time before we brutally shutdown the thread executor.