Set ReqsCollection as WorkerChange listener

Set ReqsCollection as WorkerChange listener

WorkerChange listener is notified about changes of worker state:
* when worker becomes IDLE;
* when worker's work has been broken by entering FAIL
 or MAINTENANCE state.

The ReqsCollection implementing WorkerChange interface reacts by:
* matching new pending request to IDLE worker;
* finishing request execution with FAILED state if worker's job has
 been broken.

Change-Id: Icff6a91e3173712243bf6d8ee382d1f1222b4735
Signed-off-by: Lukasz Wojciechowski <l.wojciechow@partner.samsung.com>