* feat(namespaces): Initial support for multi-tenant #3050
This PR introduces a way to create multiple, separate and isolated
namespaces in Dragonfly. Each user can be associated with a single
namespace, and will not be able to interact with other namespaces.
This is still experimental, and lacks some important features, such as:
* Replication and RDB saving completely ignores non-default namespaces
* Defrag and statistics either use the default namespace or all
namespaces without separation
To associate a user with a namespace, use the `ACL` command with the
`TENANT:<namespace>` flag:
```
ACL SETUSER user TENANT:namespace1 ON >user_pass +@all ~*
```
For more examples and up to date info check
`tests/dragonfly/acl_family_test.py` - specifically the
`test_namespaces` function.
