ReplicationTasksStorage: Add multi-primary unit tests

These tests examine the replication scenarios under
multi-primary setup making use of the api calls present
in ReplicationTasksStorage class similarly as done in
single primary setup.

These tests ensure that the replication compatability in
multi-primary setup is not broken.

Change-Id: Ib2d0017c4d2ac3f4cfc7262b68b09a3a357e1337