Split library into two parts: DB-API 2.0 compatible and Asyncio style

Nadie ha tomado este issue todavía.

I am very interested in using this library since type support in `psycopg2` is awful to work with. However, some projects still use Python 2.7, and will for another 4 years. Would it be possible to split `asyncpg` into two parts, one with an asynchronous API, usable with Python 3.5 and above, and one synchronous API that can be compatible across both 2.7 and 3.5 and above?

My proposal is a DB-API 2.0 ([PEP 249](https://www.python.org/dev/peps/pep-0249/)) compatible API that can be utilized by the `asyncio` part of the library. The benefits is that applications that do not need asynchronous calls can use the same underlying library as those who do. With this code that doesn't rely on co-routines can use the type support provided by `asyncpg`.

I haven't used this project yet, or dived into its internals, and don't know whether it is feasible at all to do this. I am just venting my ideas.

El issue no menciona archivos de implementación ni pruebas. Empieza revisando la API asyncio existente de asyncpg y comparando la interfaz síncrona DB-API 2.0 propuesta con PEP 249 y la compatibilidad con Python 2.7. Dado que se trata de una propuesta arquitectónica exploratoria, los maintainers tendrían que definir primero la viabilidad, el alcance y un estado final medible.

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