Sync connection pooling with r2d2-postgres and sync GenericClient.

[LouisGariepy]

https://github.com/sfackler/r2d2-postgres

I don't have strong opinions on this, but that way we give more flexibility to the users. Its pretty easy to implement, and it would make the client code for sync and async symmetric.

That would probably imply giving cornucopia_client sync and async features so users only import what they need.

[LouisGariepy]

See #84