Handle malformed proxy protocol header

If the proxy protocol header contains a malformatted string, such as "PROXY UNKNOWN", instead of throwing and killing the connection, use the TCP source IP as the remote IP.

Also changed how the header is read from the buffer, to avoid a potential Netty resource leak. Originally the header is read into another ByteBuf, which needs be be explicit released in order for Netty to reclaim its memory (http://netty.io/wiki/reference-counted-objects.html). Now we just read it into a byte array and let JVM GC it.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=188047084

javatests/google/registry/proxy/handler/ProxyProtocolHandlerTest.java

@@ -49,6 +49,20 @@ public class ProxyProtocolHandlerTest {
     assertThat(channel.isActive()).isTrue();
   }
 
+  @Test
+  public void testSuccess_proxyHeaderMalformed_singleFrame() {
+    header = String.format("PROXY UNKNOWN\r\n");
+    String message = "some message";
+    // Header processed, rest of the message passed along.
+    assertThat(channel.writeInbound(Unpooled.wrappedBuffer((header + message).getBytes(UTF_8))))
+        .isTrue();
+    assertThat(((ByteBuf) channel.readInbound()).toString(UTF_8)).isEqualTo(message);
+    // Header malformed.
+    assertThat(channel.attr(REMOTE_ADDRESS_KEY).get()).isNull();
+    assertThat(channel.pipeline().get(ProxyProtocolHandler.class)).isNull();
+    assertThat(channel.isActive()).isTrue();
+  }
+
   @Test
   public void testSuccess_proxyHeaderPresent_multipleFrames() {
     header = String.format(HEADER_TEMPLATE, 4, "172.0.0.1", "255.255.255.255", "234", "123");