throttle project initialization

[kanej]

We have reports of high resource usage in repositories with a large number of hardhat projects.

We currently initialize all projects in parallel. To lessen the peak load we should throttle the number of in-parallel project initializations. This will extend the time to get to a fully setup language server, but flatten the curve of cpu/memory usage.

It is an open question whether the throttling mechanism should be limited to hardhat projects or all projects (hh projects create a thread per project).