Rewrite parallel task with Pypeln

[leplatrem]

In our checks code, we use our little run_parallel() helper to run IO operations in parallel. This serves us well, but there is a limitation: the parallel tasks are «gathered» at each step of the check.

Mythmon suggested the pypeln library, which seems to allow the execution of several steps in parallel (pipes).

If exceptions are reported the same way as the current code, it should be pretty pleasant to rewrite the checks :)