PORTAL-377 Share session for Scheduler and RM

The sessions handling have been merged so the same sessionId can be used for
both the Scheduler and the RM.

Connections to the underlying Scheduler/RM via ProActive is done in a lazy manner.

Clock and SchedulerRMProxyFactory were introduced for testing.

Change-Id: I01f8bbed0378662b30da366b0e1c43fb3c65e4ee