Revert "service: replace eventlet event by threading"

This reverts commit f08f6ba7e80729fb6028b24568da0f651a75fcd6.

This was a terrible idea: as soon as you don't monkey patch threading,
this does not work as expected in the end.

So it's better to say that it's explicitly eventlet mechanisms that are
required, at least for now.

Conflicts:
	tests/unit/test_service.py

Partial-Bug: #1274044

Change-Id: I4d4f112d4550d912c4f81e62bc9a4714cf117384
Merge "Revert "service: replace eventlet event by threading""