fix: shopware messengers are orphaned on crash or SIGINT

[pkramme]

This PR does a few things for the shopware-cli project worker command:

• Shopware messengers are terminated with SIGKILL when shopware-cli terminates
• SIGINT is now catched by the signal handler, so that you can ^C correctly
• There is a new option --graceful-stop-limit, which signals messengers with SIGTERM and waits for a specified amount of time before using SIGKILL. When PCNTL is available in the messengers, this allows the messenger to finish handling their last message.
• Crashes of shopware-cli are now (mostly, hopefully) prevented by instead restarting the workers. This better handles situations where one messenger crashes for some reason, which previously detached all other messengers from shopware-cli, creating storms of messengers overloading systems