Flint: Add watchdogs

This MR introduce 2 watchdogs to allow to debug Flint and protect the beamline in production.

This have to be enabled using:

SCAN-DISPLAY.extra_args = ["--enable-watchdog"]

The first one can be triggered manually to be able to retrieve information when Flint is freezed.

kill -USR1 FLINT_PID   # 1st time: start monitoring
kill -USR1 FLINT_PID   # 2nd time: log monitoring

The second is a memory watchdog triggered according to the percentage of the memory used by Flint (the one displayed by top).

• At 50% the monitoring is started
• At 60% the data monitored is logged and it tries to close Flint
• At 70% send a kill signal

Log results can be found in /var/log/bliss/flint_ with the name of the session.