Block access to all predicates by default in ACL

Moved from GitHub dgraph/4082

Posted by ashish-goswami:

Currently if dgraph cluster is booted up with ACL on, all predicates can be accessed until first rule is created. Ideally we should not allow access to any predicates by default if ACL is on. We can expose acl_allow_all flag to allow access to all predicates after the cluster boot up.

Final behaviour will look as follows -

ACL is off

Access to all predicates is OK

ACL is on

NO Access to any predicates for reading/writing/modifying

ACL is on but cluster is run with the option acl_allow_on

Access to all predicates is OK

campoy commented :

We should indeed fix this, yes. I agree with the proposed behavior.