Connection, returned to pool, should not be usable

The following bug lived in our service for many month, once a few days producing strange error messages. I've even fixed a [real bug](https://github.com/aio-libs/aiopg/issues/216) in `aiopg` trying to figure out what is going on:
```
async with request.app["db"].acquire() as db:
await func1(db)

await func2(db)
```
Connection has been returned to pool, but is still being used outside of pool management logic. This can lead to various concurrency conflicts.

In my opinion, connections that are managed by the pool, should have some flag that either issues warning or raises exception that prevents using them this way. If community agrees, I can implement this and issue a pull-request.

The entry point shown is request.app["db"].acquire() used as an async context manager; start by tracing what happens when that context exits and the connection returns to the pool. Define and test the chosen warning or exception behavior for later use of db, including the concurrency-conflict case described.

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