Prevent one project from blocking all others to sync

[flodolo]

https://pontoon.mozilla.org/sync/log/215326/

Foundation Content took almost 2 hours to sync, blocking most of the other projects (not all of them, somehow).

Filing an issue to make sure this is on the radar, even if we talked about it before. This is likely caused by the TOML file having hundreds of entries.

I guess part of solving the issue is optimizing the performance, but the most important part (maybe the easiest?) would be making sure such a project doesn't block all the others.

[mathjazz]

Let's use this issue to fix sync task concurrency.

Slow performance of sync tasks with complex TOML files is unrelated to that and should be addressed separately.