Incomplete upgrade path for 7.10 to 8.10 migration

There is currently no upgrade path from 7.10 to 8.10 when using postgresql.

7.10 supported databases show up to version 9.4:
https://doc.nuxeo.com/710/admindoc/supported-databases/

8.10 supported databases show only 9.5:
https://doc.nuxeo.com/810/nxdoc/postgresql/

1. Either 7.10 should support 9.5 or 8.10 should support 9.4. Upgrading at the same time Nuxeo and Database is not acceptable since it is close to a big bang test.
2. One should be able to upgrade each component individually to alleviate the risk of a painful migration