ade: Detect gdbserver process termination

When the debugger thread starts, it spawns a gdbserver and
waits forever. Use the Channel from paramiko to monitor the process
return value and quit when it's terminated.
Discard any output from gdbserver as it can stall the ssh link in extreme
cases.
Additionnaly "while True: pass" loops are busy loops. Relax those.
Do that for non-interactive ade too.

Apertis: https://phabricator.apertis.org/T4462



Signed-off-by: Frédéric Dalleau <frederic.dalleau@collabora.com>
Reviewed-by: Emanuele Aina <emanuele.aina@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D7431