Add a simple DNS caching system to BNetworkAddress

netresolv (and libbind) won't cache DNS requests, which can result in a
lot of DNS requests being made for the same host. Implement a simple
cache in RAM (local to each application) which will keep the most
recently requested addresses cached. This can speed up loading of an
HTTP page a lot, by saving a DNS request for each resource stored on the
same server as the main page.

headers/os/net/NetworkAddressResolver.h

@@ -6,6 +6,9 @@
 #define _NETWORK_ADDRESS_RESOLVER_H
 
 
+#include <ObjectList.h>
+#include <Referenceable.h>
+#include <String.h>
 #include <SupportDefs.h>
 
 
@@ -20,7 +23,7 @@ enum {
 };
 
 
-class BNetworkAddressResolver {
+class BNetworkAddressResolver: public BReferenceable {
 public:
 								BNetworkAddressResolver();
 								BNetworkAddressResolver(const char* address,
@@ -53,9 +56,54 @@ public:
 			status_t			GetNextAddress(int family, uint32* cookie,
 									BNetworkAddress& address) const;
 
+	// TODO all the ::Resolve variants are needed. Maybe the SetTo and
+	// constructors could be removed as ::Resolve is the right way to get a
+	// resolver (using the cache).
+	static	BReference<const BNetworkAddressResolver> Resolve(
+									const char* address, const char* service,
+									uint32 flags = 0);
+	static	BReference<const BNetworkAddressResolver> Resolve(
+									const char* address, uint16 port = 0,
+									uint32 flags = 0);
+	static	BReference<const BNetworkAddressResolver> Resolve(int family,
+									const char* address, const char* service,
+									uint32 flags = 0);
+	static	BReference<const BNetworkAddressResolver> Resolve(int family,
+									const char* address, uint16 port = 0,
+									uint32 flags = 0);
+
 private:
 			addrinfo*			fInfo;
 			status_t			fStatus;
+
+
+	struct CacheEntry {
+		CacheEntry(int family, const char* address, const char* service,
+			uint32 flags, BNetworkAddressResolver* resolver)
+			:
+			fFamily(family),
+			fAddress(address),
+			fService(service),
+			fFlags(flags),
+			fResolver(resolver, false)
+		{
+		}
+
+		bool Matches(int family, BString address, BString service, uint32 flags)
+		{
+			return family == fFamily && flags == fFlags && address == fAddress
+				&& service == fService;
+		}
+
+		int fFamily;
+		BString fAddress;
+		BString fService;
+		uint32 fFlags;
+
+		BReference<const BNetworkAddressResolver> fResolver;
+	};
+
+	static	BObjectList<CacheEntry> sCacheMap;
 };