feat(server): make the RetryInterval of PeerServer tunable

For tests and other environments it would be nice to be able to tune how
long to sleep between retries.

server/usage.go

@@ -52,6 +52,7 @@ Peer Communication Options:
 Other Options:
   -max-result-buffer   Max size of the result buffer.
   -max-retry-attempts  Number of times a node will try to join a cluster.
+  -retry-interval      Seconds to wait between cluster join retry attempts.
   -max-cluster-size    Maximum number of nodes in the cluster.
   -snapshot=false      Disable log snapshots
   -snapshot-count      Number of transactions before issuing a snapshot.