(PCP-701) Only schedule purge when broker is running
(PCP-701) Only schedule purge when broker is running

When the broker is not running, there are guaranteed to be no clients
connected. Scheduling a purge every time the broker is HUPed could lead
to many extraneous threads. Only schedule a client purge if the broker
is already running.

puppetlabs/pcp-broker,puppetlabs/pcp-broker,puppetlabs/pcp-broker