IdentityDao: make instance() method static
IdentityDao: make instance() method static

Fixes: 2d2c29a35af9 ("IdentityDao: new class")
Signed-off-by: Jan Viktorin <3eab6ac54c005c4408f608556a6663b7f2abede7@fit.vutbr.cz>

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