An extension may want to call GetSecurityLabel() on a shared object
before the shared relcaches are fully initialized. For instance, a
ClientAuthentication_hook might want to retrieve the security label on
a role.

Discussion: https://postgr.es/m/ecb7af0b26e3be1d96d291c8453a86f1f82d9061.camel@j-davis.com
Backpatch-through: 9.6