MAJOR: http: add the keep-alive transition on the server side

MAJOR: http: add the keep-alive transition on the server side

When a connection to the server is complete, if the transaction
requests keep-alive mode, we don't shut the connection and we just
reinitialize the stream interface in order to be able to reuse the
connection afterwards.

Note that the server connection count is decremented, just like the
backend's, and that we still try to wake up waiters. But that makes
sense considering that we'll eventually be able to immediately pass
idle connections to waiters.