cannot switch to state 15; another operation (2) is in progress

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* **asyncpg version**: 0.23.0
* **PostgreSQL version**: Aurora PostgreSQL 10.14
* **Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
the issue with a local PostgreSQL install?**: RDS Aurora (and hard to repro)
* **Python version**: 3.9
* **Platform**: Linux
* **Do you use pgbouncer?**: no
* **Did you install asyncpg with pip?**: pip3.9 install --user
* **If you built asyncpg locally, which version of Cython did you use?**:
* **Can the issue be reproduced under both asyncio and [uvloop](https://github.com/magicstack/uvloop)?**: repro'd with uvloop

```
File "/root/.local/lib/python3.9/site-packages/asyncpg/pool.py", line 792, in __aexit__
await self.pool.release(con)
File "/root/.local/lib/python3.9/site-packages/asyncpg/pool.py", line 666, in release
return await asyncio.shield(ch.release(timeout))
File "/usr/lib/python3.9/asyncio/futures.py", line 284, in __await__
yield self # This tells Task to wait for completion.
File "/usr/lib/python3.9/asyncio/tasks.py", line 328, in __wakeup
future.result()
File "/usr/lib/python3.9/asyncio/futures.py", line 201, in result
raise self._exception
File "/usr/lib/python3.9/asyncio/tasks.py", line 256, in __step
result = coro.send(None)
File "/root/.local/lib/python3.9/site-packages/asyncpg/pool.py", line 218, in release
raise ex
File "/root/.local/lib/python3.9/site-packages/asyncpg/pool.py", line 208, in release
await self._con.reset(timeout=budget)
File "/root/.local/lib/python3.9/site-packages/asyncpg/connection.py", line 1311, in reset
await self.execute(reset_query, timeout=timeout)
File "lib/common.py", line 164, in execute
return await self._with_logger(super(PSQLLoggerMixin, self).execute)(*args, **kwargs)
File "lib/common.py", line 119, in wrapper
ret_val = await func(*args, **kwargs)
File "/root/.local/lib/python3.9/site-packages/asyncpg/connection.py", line 297, in execute
return await self._protocol.query(query, timeout)
File "asyncpg/protocol/protocol.pyx", line 338, in query
File "/usr/lib/python3.9/asyncio/futures.py", line 287, in __await__
return self.result() # May raise too.
File "/usr/lib/python3.9/asyncio/futures.py", line 201, in result
raise self._exception
File "asyncpg/protocol/protocol.pyx", line 331, in asyncpg.protocol.protocol.BaseProtocol.query
File "asyncpg/protocol/coreproto.pyx", line 1061, in asyncpg.protocol.protocol.CoreProtocol._simple_query
File "asyncpg/protocol/coreproto.pyx", line 768, in asyncpg.protocol.protocol.CoreProtocol._set_state
asyncpg.exceptions._base.InternalClientError: cannot switch to state 15; another operation (2) is in progress
```

Start with asyncpg/pool.py around release, asyncpg/connection.py around reset, and asyncpg/protocol/coreproto.pyx around _set_state. Review the traceback and investigate the concurrent-operation path, noting that the report is hard to reproduce and was seen with uvloop and Aurora PostgreSQL. Done means the cause is reproducible or explained and the state-switch failure is corrected without breaking connection release.

Written by the indexing model from the issue text.