MINOR: connection: provide a generic data layer wakeup callback

MINOR: connection: provide a generic data layer wakeup callback

Instead of calling conn_notify_si() from the connection handler, we
now call data->wake(), which will allow us to use a different callback
with health checks.

Note that we still rely on a flag in order to decide whether or not
to call this function. The reason is that with embryonic sessions,
the callback is already initialized to si_conn_cb without the flag,
and we can't call the SI notify function in the leave path before
the stream interface is initialized.

This issue should be addressed by involving a different data_cb for
embryonic sessions and for stream interfaces, that would be changed
during session_complete() for the final data_cb.