tokio-postgres: add execute_prepared function

[mahkoh]

As it says in the documentation:

This function is identical to execute_raw except that:

1. The returned future does not borrow the parameters or self.
2. The type of the returned future does not depend on the parameters.
3. If multiple such futures are being used concurrently, then they are
   executed on the server in the order in which this function was
   called, regardless of the order in which the futures are polled.
4. This function can only be used with prepared statements.

This has the following additional advantages:

1. Multiple non-homogeneous execute futures can be stored in a collection without having to box the futures.
2. The parameters can be temporaries that do not need to out-live the futures.
3. We can rely on the order in which the futures reach the server without having to rely on polling order which might not always be documented.
4. It is not necessary to poll the returned futures at all. An application can also only poll the final transaction commit.