feat(namespaces): Initial support for multi-tenant (#3260)

* 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.

src/server/main_service.h

@@ -102,7 +102,7 @@ class Service : public facade::ServiceInterface {
   }
 
   // Used by tests.
-  bool IsLocked(DbIndex db_index, std::string_view key) const;
+  bool IsLocked(Namespace* ns, DbIndex db_index, std::string_view key) const;
   bool IsShardSetLocked() const;
 
   util::ProactorPool& proactor_pool() {