[RELEASE] Released version 1.5.6

Released version 1.5.6 with the following main changes :
    - BUG/MEDIUM: systemd: set KillMode to 'mixed'
    - MINOR: systemd: Check configuration before start
    - BUG/MEDIUM: config: avoid skipping disabled proxies
    - BUG/MINOR: config: do not accept more track-sc than configured
    - BUG/MEDIUM: backend: fix URI hash when a query string is present