evmgr: Isolate the event loop from jobs

The event loop is renamed evmgr for "event manager"
with the intention (1) still use evloop in jobs (2)
to provide an abstract event manager/handler/loop
abstraction.

Change-Id: Ib1955f661f98df80e1c7be99e9fe26a1e06d78f6
Signed-off-by: José Bollo <jose.bollo@iot.bzh>