MEDIUM: checks: close the socket as soon as we have a response

MEDIUM: checks: close the socket as soon as we have a response

Until now, the check socked was closed in the task which handles the
check, which can sometimes be substantially later when many tasks are
running. It's much cleaner to close() in the wake call, which also
helps removing some FD management from the task itself.

The code is faster and smaller, and fast health checks show a more
predictable behaviour.