Make heartbeat errorCount non-absolute.

This patch allows for periodic hiccups to occur (due to system load) during updating a services TTL in skydns.  Each time an update is made without error, the error count will decrease by one.  Only after 10 sequential errors will the heartbeat goroutine exit out.

To take advantage of the ability to punch through periods of high system load use a slightly higher TTL e.g. -ttl 120 seconds and a lower beat e.g. -beat 15 seconds.

A longer term solution would be to check that the service is still registered in skydns using skydns.Get(uuid) and re-register the service if it is missing instead of assuming it hasn't timed out already.