MockRelationDao: introduce new class
MockRelationDao: introduce new class

Represent DAO class for objects with a strong relation to
a base object. E.g. devices are always assigned to a gateway.
There can be more devices of the same ID assigned to different
gateways.

Signed-off-by: Jan Viktorin <3eab6ac54c005c4408f608556a6663b7f2abede7@fit.vutbr.cz>

BeeeOn/server,BeeeOn/server,BeeeOn/server,BeeeOn/server