Cursor timeout should not match connection timeout

It appears that the cursor timeout, when set to None, will always take the connection timeout as the maximum amount of time a query can run.

I believe this is a bug and should be changed.

https://github.com/aio-libs/aiopg/blob/master/aiopg/connection.py#L260

If this is not a bug and the intended behavior, it is very opinionated and should be documented.

A connection timeout is for establishing a connection to the database and it is very possible to have long running queries that exceed the connection timeout.

At the very least, I would like a way to disable the cursor timeout all together, but it appears that is not possible.

Start at aiopg/connection.py around line 260 and trace how a cursor timeout of None is derived from the connection timeout. Reproduce the behavior with a long-running PostgreSQL query, then verify that the completed change either separates the timeouts or clearly documents the intended behavior and provides a way to disable cursor timeouts.

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