Change PostgreSQL authentication method to scram-sha-256

Since (https://jdbc.postgresql.org/documentation/changelog.html#version_42.2.0) JDBC 42.2.0 and PostgreSQL 10 we can use SCRAM-SHA-256 authentication method.

As we can see in https://www.postgresql.org/docs/current/auth-password.html this method is:

This is the most secure of the currently provided methods, but it is not supported by older client libraries.

Until now, in a instance based on 6.2 (or default branch) with SCRAM-SHA-256 no problems were detected.

Maybe in future, with more feedback from community we can change installation guides to use this method and provide more security in a custom install for new users.

For reference, to change an existing instance you need:

1) modify your postgresql.conf:
password_encryption = scram-sha-256

2) change pg_hba.conf auth method from md5 to scram-sha-256
TYPE DATABASE USER ADDRESS METHOD
host all all 1.2.3.4/32 scram-sha-256

3) Reload your database to apply new configurations
select pg_reload_conf();

4) Change users password:
alter user postgres with password 'add_new_password';

NOTE: to get a list with all users that passwords should be changed use:
SELECT rolname FROM pg_authid WHERE rolcanlogin;