support database-level isolation

[fuyufjh]

Is your feature request related to a problem? Please describe.

Currently, tables in databases are not isolated.

$ psql -h localhost -p 4566 db1
psql (14.2, server 9.5.0)
Type "help" for help.

db1=> show tables;
   Name
----------
 region
 nation
 part
 orders
 lineitem
 customer
 supplier
 partsupp
(8 rows)


$ psql -h localhost -p 4566 db2
psql (14.2, server 9.5.0)
Type "help" for help.

db2=> show tables;
   Name
----------
 region
 nation
 part
 orders
 lineitem
 customer
 supplier
 partsupp
(8 rows)

Describe the solution you'd like

Every database should act like an isolated instance - objects like tables and indexes should be isolated between databases.

Also, please check Postgres for detailed behaviors.

Describe alternatives you've considered

Additional context

[hzxa21]

Related issue #2529