Ensure all tickers are explicitly stopped.

As pointed out in #189, according to the Go documentation, a ticker must
be stopped to release associated resources.  This commit adds a defer call
to stop two tickers there were previously not being stopped as well as
changes two others that were being stopped over to use defer so it's more
consistent.

The other ticker in ScheduleShutdown is replaced and already calls Stop
before replacing it, so it has not been modified.

Closes #189.

ok @jrick