node cache: expire nodes on destructions

Expiring nodes on cache destruction allow to cancel
any remaining request before the dht instance is destructed,
since Node instances might remain alive after the dht instance
is destroyed.

src/node_cache.cpp

@@ -20,6 +20,12 @@
 
 namespace dht {
 
+NodeCache::~NodeCache()
+{
+    cache_4.setExpired();
+    cache_6.setExpired();
+}
+
 Sp<Node>
 NodeCache::getNode(const InfoHash& id, sa_family_t family) {
     return cache(family).getNode(id);
@@ -116,4 +122,12 @@ NodeCache::NodeMap::clearBadNodes() {
     }
 }
 
+void
+NodeCache::NodeMap::setExpired() {
+    for (auto& wn : *this)
+        if (auto n = wn.second.lock())
+            n->setExpired();
+    clear();
+}
+
 }