When Raft wants a snapshot (MsgSnap), we (on the leader) .Add the range to the raft snapshot queue. However, there are various code paths that can lead to the range not receiving a snapshot "right away" (purgatory, max size limitation). If that happens, the Raft group is stuck in an awkward state until the scanner picks up the range again and hopefully does give it a snapshot.

This setup seems risky and we may want to address the above points so that the Raft-initiated snapshots have no chance of being lost.