Delegate asyncpg

[fantix]

Currently, GINO, SQLAlchemy and asyncpg works together in a way that user needs to manually take care of the three - create a GINO environment, define columns with SQLAlchemy, create asyncpg pools and feed connection object to GINO operations. Which is fine, it is expected to be used like this, as the low-level interface.

However, if GINO could delegate asyncpg interface, user code could be very much simplified - GINO shall be able to map the rows to the correct model automatically for example. Let's try to make such delegation, in an optional way on top of the GINO low-level interface.