jobs: Refactor event loop handling

This improves the arbitration of the single event
loop across threads.

Before introduction of using 'evenfd' there was
several event loop. At the current time, there
is only one. At the end, there will probably
remain only one.

Bug-AGL: SPEC-2089

Change-Id: Iac9db7cbe15b4c9c76e6e9a8f6e641ed2a9039e0
Signed-off-by: José Bollo <jose.bollo@iot.bzh>