Milkcheck should process targets independently

[wilfriedroset]

Some services may need to process all targets at once but others don't. For those who don't need to process targets at once, milkcheck should handle targets independently.

Let's say we have 3 nodes and 1 of them is slower for some reason (but faster than the timeout), we need to execute 4 commands on them.

Currently milkcheck would do:

• cmd1 on nodes[1-3], wait for the slower of cmd1 (blocking)
• cmd2 on nodes[1-3], wait for the slower of cmd2 (blocking)
• ...

It would be more efficient to work like that:

• cmd1 on nodes[1-3], (in background wait for the slower of cmd1, not blocking)
• Launch cmd2 as soon as targets are done with cmd1, (in background wait for the slower of cmd1 et cmd2, not blocking)
• ...

This way, we will use more efficiently the fanout instead of waiting each time for the slower targets.
This issue/feature is related to issue #6