Use (and add support for) prefer as the default SSL mode on the Postgres and MySQL connectors

Which use case/requirement will be addressed by the proposed feature?

Configuration item database.sslmode on the Postgres and MySQL connectors has a default value of disable. It would be good for the connection to the Postgres/MySQL instance to be secure by default (or at least attempt to be encrypted).

Implementation idea

Postgres

The supported values for database.sslmode are disable, require, verify-ca and verify-full. In In addition to those values, the Postgres JDBC [driver|https://jdbc.postgresql.org/documentation/publicapi/org/postgresql/jdbc/SslMode.html] supports 2 more values:

• allow: try to connect to Postgres without encryption and, failing that, try encrypted 
• prefer: try to connect to Postgres with encryption and, failing that, try unencrypted

Let's add support for both of those modes and then make the prefer value the default mode.  

MySQL

The support values for database.sslmode are disabled, preferred, required, verify_ca and verify_identify. The default is disabled. Let's make preferred the default mode.