Remember that a replaced downstairs is replaced (#1426)

Leave a downstairs in state Replaced after a replacement request until
we connect to the new downstairs and either validate it is correct or
determine it is bad.

This fixes a bug where a replaced downstairs would go to failed, then
cause problems when we eventually connect to that downstairs.

Fix for: #1425

upstairs/src/client.rs

@@ -532,8 +532,7 @@ impl DownstairsClient {
 
             DsState::Faulted
             | DsState::LiveRepair
-            | DsState::LiveRepairReady
-            | DsState::Replaced => DsState::Faulted,
+            | DsState::LiveRepairReady => DsState::Faulted,
 
             DsState::New
             | DsState::Deactivated
@@ -546,7 +545,8 @@ impl DownstairsClient {
             | DsState::WaitActive
             | DsState::Disabled => DsState::Disconnected,
 
-            DsState::Replacing => DsState::Replaced,
+            // If we have replaced a downstairs, don't forget that.
+            DsState::Replacing | DsState::Replaced => DsState::Replaced,
 
             DsState::Migrating => panic!(),
         };