bug#43564: cuirass: Contention while registering new builds.

[Mathieu Othacehe]

Hello,

> In the best case of one pending evaluation, the registration duration is
> reduced from 1800 seconds to 320 seconds. I think that the gain is way
> larger when there are multiple pending evaluations.
>
> Pushed a fix as 461e07e14e1c8013343c0a2cb26c0e022e10d5e4.

While this improves the situation, another evaluation contention episode
occurred this afternoon (evaluations 16920 to 16938).

Even though reading queries are cheaper, I guess that when 10 fibers try
to execute 50k read queries, there's some contention in the
communication with the 4 database workers.

I'm trying locally to spawn database workers dedicated to registration,
that execute the evaluation queries in a single batch. Let's see if it
helps.

Thanks,

Mathieu