discoverd/client: Add jitter to heartbeats
discoverd/client: Add jitter to heartbeats

Each heartbeat should happen 3-5 seconds after the last successful one or
200-400ms after the last failing heartbeat.

Signed-off-by: Jonathan Rudenberg <3692bfa45759a67d83aedf0045f6cb635a966abf@titanous.com>

lmars/flynn,flynn/flynn,philiplb/flynn,flynn/flynn,flynn/flynn,philiplb/flynn,flynn/flynn,lmars/flynn,philiplb/flynn,philiplb/flynn,philiplb/flynn,lmars/flynn,flynn/flynn,lmars/flynn,lmars/flynn