Possibly use SQLAlchemy Strategies

It looks like the `aiopg.sa` module relies on reimplementing some of the lower level parts of SQLAlchemy and providing alternatives which use a similar API. I wonder if this could instead be done using SQLAlchemy's support for different execution strategies?

There is an example of doing this for Twisted at https://github.com/alex/alchimia, that defers things to a thread pool, largely because it wants to work for any database not just PostgreSQL. However I wonder if it would be possible to do that instead which might deduplicate some of the code?