Implement WorkersManager interface in WorkerList

Implement WorkersManager interface in WorkerList

Implementation of WorkersManager from matcher package makes WorkerList usable
as interface for acquiring workers by Matcher.

The implemnetation adds 2 new fields:
* changeListener which is notified after Worker's state changes;
* newDryadClient which provides dryad.ClientManager.

The implementation is covered by tests.
2 mock up types: MockDryadClientManager and MockWorkerChange for mocking up
rpc.dryad.ClientManager and WorkerChange.

Change-Id: I2177824aef7aea564cd0a9900d9970c8a8386ca7
Signed-off-by: Lukasz Wojciechowski <l.wojciechow@partner.samsung.com>