Currently, when reloading the Tree, async progress indicators are lost.

2 solutions are possible:

1. backend route to give all current tasks for a given Tree
2. frontend tree memory to remember which async task have to be reloaded

For now, 1) is tricky for some reasons but would be good for long term (not now)

For now, 2) is quicker if we save locally the node and route to fetch job status (and remove it if job is done only).

(This bug is related with #137 (closed) )