catalog subsystem should cope with being called with different lua states
whelk 0.3.2 was assuming that lua state would always be the same. If it
wasn't then there would be no cat admin metatable registered in the second
lua state but whelk would just assume that there was causing things to go
wrong. We now use luaL_newmetatable() to test for whether a metatable was
already registered and take the appropriate actions if it wasn't.