* storage/raft: Fix cluster init with retry_join
Commit 8db66f4853abce3f432adcf1724b1f237b275415 introduced an error
wherein a join() would return nil (no error) with no information on its
channel if a joining node had been initialized. This was not handled
properly by the caller and resulted in a canceled `retry_join`.
Fix this by handling the `nil` channel respone by treating it as an
error and allowing the existing mechanics to work as intended.
* storage/raft: Improve retry_join go test
* storage/raft: Make VerifyRaftPeers pollable
* storage/raft: Add changelog entry for retry_join fix
* storage/raft: Add description to VerifyRaftPeers
