Async run_transaction or aiopg and asyncpg

[tailhook]

Would be nice to have, and looks easy enough to implement.

Eventually, I'll probably implement one myself if nobody jumps in. Any hints for implementation?

[tailhook]

Also, it looks like transaction.py imports psycopg2 for no reason (just to catch error name), if I use asyncpg. So the the line:

from .transaction import run_transaction  # noqa

... should be removed. Or module itself should be made more robust.