BUG/MEDIUM: connection: Don't consider new private connections as available

BUG/MEDIUM: connection: Don't consider new private connections as available

When a connection is created and the multiplexer is installed, if the connection
is marked as private, don't consider it as available, regardless the number of
available streams. This test is performed when the mux is installed when the
connection is created, in connect_server(), and when the mux is installed after
the handshakes stage.

No backport needed, this is 2.2-dev.