Unblock apply future during shutdown.

[ncabatoff]

Attempt at a fix for #498.

[stale]

Hey there,
We wanted to check in on this request since it has been inactive for at least 90 days.
Have you reviewed the latest godocs?
If you think this is still an important issue in the latest version of the Raft library or
its documentation please feel let us know and we'll keep it open for investigation.
If there is still no activity on this request in 30 days, we will go ahead and close it.
Thank you!