MEDIUM: checks: Stop scheduling healthchecks during stopping stage

MEDIUM: checks: Stop scheduling healthchecks during stopping stage

When the process is stopping, the health-checks are suspended. However the
task is still periodically woken up for nothing. If there is a huge number
of health-checks and if they are woken up in same time, it may lead to a
noticeable CPU consumption for no reason.

To avoid this extra CPU cost, we stop to schedule the health-check tasks
when the proxy is disabled or stopped.

This patch should partially solve the issue #2145.