ReplicationTasksStorage.Task: Add multi-primary unit tests

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

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

Change-Id: I980e8286bf11d31c6ab89e49ef065fdde1118181