Refactor store interfaces and provide database as example implementation

controller/mocks/Transport.go

controller/northbound/server/core.go

controller/northbound/server/csbi.go

controller/northbound/server/nbi.go

controller/northbound/server/pnd.go

controller/northbound/server/pnd_test.go

controller/northbound/server/sbi.go

controller/nucleus/database/mongo-connection.go

controller/nucleus/device.go

controller/nucleus/deviceStore.go

controller/nucleus/errors/errors.go

controller/nucleus/genericStore.go

controller/nucleus/initialise_test.go

controller/nucleus/memoryPndStore.go

controller/nucleus/pndStore.go

controller/nucleus/principalNetworkDomain.go

controller/nucleus/principalNetworkDomain_test.go

controller/nucleus/sbiStore.go

controller/nucleus/southbound.go

controller/store/change_store_test.go.wasd