MEDIUM: connection: add an ->init function to data layer

MEDIUM: connection: add an ->init function to data layer

SSL need to initialize the data layer before proceeding with data. At
the moment, this data layer is automatically initialized from itself,
which will not be possible once we extract connection from sessions
since we'll only create the data layer once the handshake is finished.

So let's have the application layer initialize the data layer before
using it.