Better access control in comdb2

The access control should be enhanced. I am adding a few ideas here:
1) we need a user abstraction, that will aggregate user ids (established through SSL certificates or username&password, for example), and credentials
2) credentials are tuples `(userid, tablename, accesstype)`; access type could be read, write, and so on. Table name could be a local table, for example `"t1"`, or a remote table `"dbname1.t2"`. We could accept wildcards to make things simple, for example `"*"` means everything, `"dbname2.*"` means every table in `dbname2`. and so on
3) For simplicity, we could handle access rights at the ingress point (the db that the client connects to); we could enforce both local and remote access here. This eliminates the need to manage distributed keys accross multiple clusters.