Panic getting logs after restoring from an older snapshot

[fahadullah]

https://gist.github.com/fahadullah/6089c60bf6c57da0143b

When we restore a snapshot, the lastApplied index is set to the snapshot index. If we can't restore from the latest snapshot, lastApplied index is set to an old value, X. As we had a successful snapshot afterwords, append log doesn't have log entries from X+1 to the last index and it panics looking up the missing entries.

I would have assumed we would move the applied index forward or fail to restart completely in this case instead of failing later.

[slackpad]

@fahadullah thanks for opening an issue and providing a test - we will take a look at this one!