dht: fix persisted permanent values never expiring

When values are exported and re-imported, permanent values retain their
time_point::max() expiration but lose their permanent flag. This caused
them to be stored as non-permanent with an effectively infinite
expiration, so they would never expire.

Cap the expiration of non-permanent values to created + type.expiration,
ensuring that imported permanent values get a finite expiration matching
their value type.

src/dht.cpp

@@ -1432,6 +1432,8 @@ Dht::storageStore(const InfoHash& id,
     created = std::min(created, now);
     if (expiration == time_point::min())
         expiration = permanent ? time_point::max() : created + getType(value->type).expiration;
+    else if (not permanent)
+        expiration = std::min(expiration, created + getType(value->type).expiration);
     if (expiration < now)
         return false;