Implement connection/cursor procedures as generators

Implement connection/cursor procedures as generators

Generators are common to both sync and async code so there is much less
code duplication, which will be useful in the light of doing more
complex things such as prepared transactions.

As a side effect of the refactoring, operations on closed connections
are raised as OperationalError rather than InterfaceError because
connection closure may depend on external factors.

Dropped fd from PQGen stream: the having the fd in the middle of the
`Ready` values made more difficult to chain more than one operation in
the same generator. The fd is not passed as extra parameter.