Thread lock when setting constance attributes while using signals

[ArujReis]

Describe the problem

When accessing Constance attributes during the handling of a signal spawned by setting a Constance attribute, the app gets thread-locked.

Steps to reproduce

1. Set Constance to use in-memory DB
2. Create Constance config with attr_1 and attr_2
3. Create a receiver for the config_updated Constance event.
   1. In the receiver, access attr_2.
4. Change attr_1 in code, for example, in a test. (config.attr_1='test')
5. The app will hang because of a double call to self.lock in the memory backend.

System configuration

• Django version: 2.2.18
• Python version: 3.7.5
• Django-Constance version: 2.8