(MODULES-4158) Grant SELECT, UPDATE perms on sequences

Add support for granting SELECT and UPDATE permissions on sequences
(MODULES-4158).

This requires a significant change to the unless statement used to
determine whether the permission has already been granted. The previous
statement used the standard SQL information_schema tables, however these
provide limited information on sequence permissions. In order to retrieve
the current permission set with the necessary level of detail we must query
the pg_class system catalog and parse out the permisions from there.