Improve mutex lock logic.

Improve mutex lock logic.

- Use of bracket instead of unlock method
- Change some mutex lock scope.
- Added subscribed_signals map object mutex to
manipulate it safely.

Change-Id: I770c0b5701db6b1151511f7360ec31ae6dcc1de9
Signed-off-by: Romain Forlot <romain.forlot@iot.bzh>