Implement cpipe_destroy() and cbus_endpoint_destroy(). cpipe_destroy()
send a poison message to endpoint and flushes all queued. Each cbus_endpoint
counts the number of connected pipes and doesn't destroy if there are
connected pipes or unfetched messages.

cbus_endpoint_create() returns an error instead of panic in case of a
duplicate endpoint. A new endpoint can be created with same name while older
endpoint still destroing.

Fiber pool destroys endpoint, resets idle timeout to 0 and gathers all
workers.