Handle job signals from systemd

Handle job signals from systemd

A set of currently running jobs was added. Jobs are identified
with unique job path created by systemd.
Every time a start/stop/restart command is sent to systemd,
related job is added to set.
When JobNew signal is received its handler puts it also in the set.
Job is removed from the set, when JobRemoved signal is delivered from
systemd.
After each action generating a job in systemd unit (start/stop/restart)
program waits until all jobs from the set are removed.

Timeouts used earlier to "synchronize" action calls are removed.

Change-Id: Id944c9f52ecfeb06efabdb9424c835d09af7bb4f