ConnectionDoesNotExistError when releasing

Nobody has claimed this yet.

* **asyncpg version**: 0.21.0
* **PostgreSQL version**: 10.7
* **Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
the issue with a local PostgreSQL install?**: no
* **Python version**: 3.7
* **Platform**: debian (docker image `python:3.7.9-slim`)
* **Do you use pgbouncer?**: no
* **Did you install asyncpg with pip?**: yes
* **If you built asyncpg locally, which version of Cython did you use?**: no
* **Can the issue be reproduced under both asyncio and
[uvloop](https://github.com/magicstack/uvloop)?**: only tried asyncio

I've found the following exception in our logs:
```
Traceback (most recent call last):
File "/usr/local/lib/python3.7/site-packages/asyncpg/pool.py", line 214, in release
self._con.terminate()
AttributeError: 'NoneType' object has no attribute 'terminate'

During handling of the above exception, another exception occurred:
File "/usr/local/lib/python3.7/site-packages/my_code_calling_release.py", line 40, in acquire
await self.pool.release(con)
File "/usr/local/lib/python3.7/site-packages/asyncpg/pool.py", line 654, in release
return await asyncio.shield(ch.release(timeout))
File "/usr/local/lib/python3.7/site-packages/asyncpg/pool.py", line 216, in release
raise ex
File "/usr/local/lib/python3.7/site-packages/asyncpg/pool.py", line 206, in release
await self._con.reset(timeout=budget)
File "/usr/local/lib/python3.7/site-packages/asyncpg/connection.py", line 1137, in reset
await self.execute(reset_query, timeout=timeout)
File "/usr/local/lib/python3.7/site-packages/asyncpg/connection.py", line 295, in execute
return await self._protocol.query(query, timeout)
File "asyncpg/protocol/protocol.pyx", line 316, in query
```

From what I can see, the call at `self._con.reset` is raising a `ConnectionDoesNotExistError`.
Should the exception be reraised at line 218 if the exception is an `ConnectionDoesNotExistError` (we know it's already closed)? It shouldn't even try to `self._con.terminate()` because `self._con` is already None (see the exception).

Not sure if the comments in that code still apply for that type of exception

Read the release path around lines 206-218 of asyncpg/pool.py and the reset path in asyncpg/connection.py, focusing on how ConnectionDoesNotExistError leaves self._con as None. Reproduce or add a regression case for releasing an already-closed connection, then verify that release does not attempt to terminate a missing connection and preserves the intended exception behavior.

Written by the indexing model from the issue text.