Run probes asynchronously

Modify the executor to schedule each probe independently, spreading
them all over the probing interval. This reduces the risk of being
throttled by server-side rate-limiters and allows us to handle a
larger number of probes.
2 jobs for master in 2 minutes and 6 seconds (queued for 1 second)
Status Job ID Name Coverage
  Docker Build
passed #15310
docker_build

00:01:18

 
  Release
passed #15311
release

00:00:47