cmd/stdiscosrv: Record time of failed lookup

So that we can eventually garbage collect keys that noone is asking
about any more.

cmd/stdiscosrv/database.proto

@@ -14,10 +14,13 @@ option (gogoproto.goproto_getters_all) = false;
 
 message DatabaseRecord {
     repeated DatabaseAddress addresses = 1 [(gogoproto.nullable) = false];
-    int32                    misses    = 2; // Number of lookups without hits
+    int32                    misses    = 2; // Number of lookups* without hits
     int64                    seen      = 3; // Unix nanos, last device announce
+    int64                    missed    = 4; // Unix nanos, last* failed lookup
 }
 
+// *) Not every lookup results in a write, so may not be completely accurate
+
 message ReplicationRecord {
     string                   key       = 1;
     repeated DatabaseAddress addresses = 2 [(gogoproto.nullable) = false];