Add `connection_closed` or `pre_connection_closed` and `post_connection_closed` signals

[Tomasz Wójcik]

Currently I'm facing an issue with stale DB connections. It's probably related to my setup. Having connection_closed signal would simplify my debugging process.

Since 2009, there's connection_created signal.

​https://github.com/django/django/blame/6f38697f90a14f1450a71c1e40aea0f5df7dca86/django/db/backends/signals.py

I'd like to create a connection_closed signal and connect it here

​https://github.com/django/django/blob/6f38697f90a14f1450a71c1e40aea0f5df7dca86/django/db/backends/base/base.py#L314

This way the connection can be accessed right before closing it.

I'd be happy to implement it. Let me know what you think.

I'm also open to pre_connection_closed and post_connection_closed. Even though not many people will ever use it, I can imagine wanting both in different cases.