Have hosts file support for DnsNameResolver, close netty#4074

Motivation:

On contrary to `DefaultNameResolver`, `DnsNameResolver` doesn't currently honor hosts file.

Modifications:

* Introduce `HostsFileParser` that parses `/etc/hosts` or `C:\Windows\system32\drivers\etc\hosts` depending on the platform
* Introduce `HostsFileEntriesResolver` that uses the former to resolve host names
* Make `DnsNameResolver` check his `HostsFileEntriesResolver` prior to trying to resolve names against the DNS server
* Introduce `DnsNameResolverBuilder` so we now have a builder for `DnsNameResolver`s
* Additionally introduce a `CompositeNameResolver` that takes several `NameResolver`s and tries to resolve names by delegating sequentially
* Change `DnsNameResolver.asAddressResolver` to return a composite and honor hosts file

Result:

Hosts file support when using `DnsNameResolver`.
Consistent behavior with JDK implementation.

common/src/main/java/io/netty/util/internal/ObjectUtil.java

@@ -32,4 +32,48 @@ public static <T> T checkNotNull(T arg, String text) {
         }
         return arg;
     }
+
+    /**
+     * Checks that the given argument is strictly positive. If it is, throws {@link IllegalArgumentException}.
+     * Otherwise, returns the argument.
+     */
+    public static int checkPositive(int i, String name) {
+        if (i <= 0) {
+            throw new IllegalArgumentException(name + ": " + i + " (expected: > 0)");
+        }
+        return i;
+    }
+
+    /**
+     * Checks that the given argument is strictly positive. If it is, throws {@link IllegalArgumentException}.
+     * Otherwise, returns the argument.
+     */
+    public static long checkPositive(long i, String name) {
+        if (i <= 0) {
+            throw new IllegalArgumentException(name + ": " + i + " (expected: > 0)");
+        }
+        return i;
+    }
+
+    /**
+     * Checks that the given argument is positive or zero. If it is, throws {@link IllegalArgumentException}.
+     * Otherwise, returns the argument.
+     */
+    public static int checkPositiveOrZero(int i, String name) {
+        if (i < 0) {
+            throw new IllegalArgumentException(name + ": " + i + " (expected: >= 0)");
+        }
+        return i;
+    }
+
+    /**
+     * Checks that the given argument is neither null nor empty.
+     * If it is, throws {@link NullPointerException} or {@link IllegalArgumentException}.
+     * Otherwise, returns the argument.
+     */
+    public static <T> T[] checkNonEmpty(T[] array, String name) {
+        checkNotNull(array, name);
+        checkPositive(array.length, name + ".length");
+        return array;
+    }
 }

resolver-dns/src/main/java/io/netty/resolver/dns/DnsAddressResolverGroup.java

@@ -81,7 +81,11 @@ protected AddressResolver<InetSocketAddress> newResolver(
             EventLoop eventLoop, ChannelFactory<? extends DatagramChannel> channelFactory,
             InetSocketAddress localAddress, DnsServerAddresses nameServerAddresses) throws Exception {
 
-        return new DnsNameResolver(eventLoop, channelFactory, localAddress, nameServerAddresses)
+        return new DnsNameResolverBuilder(eventLoop)
+                .channelFactory(channelFactory)
+                .localAddress(localAddress)
+                .nameServerAddresses(nameServerAddresses)
+                .build()
                 .asAddressResolver();
     }
 }