[FIX] dbmanager: backup support both zip and pg_dump custom format

- let the user choose between the pg_dump custom format or the zip format including the filestore
- use file objects to allow dumps larger than memory
- postgres subprocess invocation is now clean and thread-safe, we dont touch the local process environ anymore
- add a manifest to the zip dump format with version information about odoo, postgres (pg_dump doesnt output it) and modules