common: Use a textual length when sending frames in stream

When sending messages in a stream, use a textual length frame so
that the result is more debuggable, loggable etc.

Fixes #1589
Closes #1599
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
bridge: Don't put an extra new line in init message

Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
bridge: Interactive mode for debugging the protocol

This lets use use a boundary delimiter between messages and makes
debugging the protocol easier.

Closes #1603
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>