implement echoing

Signed-off-by: Mattias Andrée <maandree@operamail.com>
mds-echo: include message id + do not require respawn status

Signed-off-by: Mattias Andrée <maandree@operamail.com>
m + fix: include header value in interception conditions

Signed-off-by: Mattias Andrée <maandree@operamail.com>