[Qgis-psc] Snap cache improvements grant report

[Julien Cabieces]

Hi PSC, devs, 

Here is my report for the Snap cache improvements grant proposal. 

We write a blog article to better explain what Snap cache improvement was about: https://oslandia.com/en/2020/01/10/en-qgis-snapping-improvements/ 

This work leads to these PRs : 
- Manage circular dependencies: https://github.com/qgis/QGIS/pull/30947
- Parallelize and relax snap index building: https://github.com/qgis/QGIS/pull/31648
- Fix QgsTaskManager waitForFinished : https://github.com/qgis/QGIS/pull/32838

The first one is available in 3.10 release while the other ones are only in master. At first, I wanted to backport them in 3.10 but I change my mind. Indeed some of the modifications are quite critical (In QgsTaskManager for instance) and are difficult to fine test (because it's all threads and synchronisation). 

We're going to invest more time and keep working on this in the days to come in order to improve stability and maybe efficiency. 

As a consequence, parallel snap cache building and relaxed mode will
land in 3.12 release. 

Thanks QGIS.org for funding this, 

Regards,
Julien