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: I1b40a11eee495cccfd3ed7a25ae6889042abd6bb
Signed-off-by: José Bollo <jose.bollo@iot.bzh>