There was a hard-to-reproduce bug in our snapshot application code.
We always compact the raft log before applying the snapshot, because
the snapshot replaces the entries and some of the logic in raft-rs
seems to rely on this. The problem was, that our compact_log function
would not remove any unapplied entries, which makes sense for compaction
triggered automatically by raft log size, but doesn't make sense for
raft snapshot, as the snapshot contains the state corresponding to the
newer entries. The fix is simple: don't guard from unapplied entry
compaction in case the compaction is for raft snapshot.
We don't add any regression tests for this, because the implementation
would be too difficult and would need us to pollute the code with error
injection logic, which is not a worthy trade off in this case. But also
the logic will still be tested, because this bug was responsible for a
large amount of flaky tests, so we should see a significant reduction in
flakiness from now on in tests concerning raft snapshots.
