Investigate whether pm can block

[xaprb]

One of our servers (no details can be given here in public) experienced some swapping and started to get into trouble. While looking at it, I did some curl calls to the pm port's /procs/ endpoint. These blocked for several minutes. I am not sure that anything is wrong with the code; this could have just been due to the swapping negatively affecting the process as a whole. But it might be a good idea to make some benchmark tests, which work like this:

• add some large N of processes to the pm, such as 10000
• benchmark how long it takes to add a new process with Start()
• benchmark how long it takes to remove with Done() and update with Status()