Implement lazy cache cleanup for DNS entries

This commit replaces the timer based approach for purging the expired
dns cache with lazy cleanup approach. The DNS cache is now cleaned
before adding a new DNS entry and before performing a lookup.

This change simplifies the code and eliminates the need for a separate
cleanup timer.

lib/dns.js

@@ -134,21 +134,13 @@ function onlookupall(err, addresses) {
   }
 }
 
-function cleanDnsCache() {
+function cleanExpiredDnsCache() {
+  const currentTime = DateNow();
   for (const key in dnsCache) {
-    delete dnsCache[key];
-  }
-}
-
-function scheduleCleanup() {
-  if (cleanupTimeout) {
-    return;
+    if (currentTime - dnsCache[key].timestamp >= 1000) {
+      delete dnsCache[key];
+    }
   }
-
-  cleanupTimeout = setTimeout(() => {
-    cleanDnsCache();
-    cleanupTimeout = null;
-  }, 1000).ref();
 }
 
 function addDnsEntry(cacheKey, address, family) {
@@ -158,7 +150,7 @@ function addDnsEntry(cacheKey, address, family) {
     timestamp: DateNow(),
   };
 
-  scheduleCleanup();
+  cleanExpiredDnsCache();
 
   return dnsCache[cacheKey];
 }
@@ -175,6 +167,7 @@ function lookup(hostname, options, callback) {
   let all = false;
   let verbatim = getDefaultVerbatim();
 
+  cleanExpiredDnsCache();
 
   // Parse arguments
   if (hostname) {
@@ -248,7 +241,7 @@ function lookup(hostname, options, callback) {
   const cacheKey = `${hostname}_${family || 'default'}`;
   const cachedResult = dnsCache[cacheKey];
 
-  if (cachedResult && DateNow() - cachedResult.timestamp < 1000) {
+  if (cachedResult) {
     callback(null, cachedResult.address, cachedResult.family);
     return {};
   }