Ignore SslHandshakeCompletionEvent (line#1874)

Motivation:

Since Netty 4.1.37, `SslHandler` emits an `SslHandshakeCompletionEvent`
when a TLS handshake is complete. It triggers a log message at WARN
level currently.

Modifications:

- Add `SslHandshakeCompletionEvent` to the known user events.

Result:

- Less noise

core/src/main/java/com/linecorp/armeria/client/HttpSessionHandler.java

@@ -49,6 +49,7 @@
 import io.netty.handler.codec.http2.Http2ConnectionPrefaceAndSettingsFrameWrittenEvent;
 import io.netty.handler.codec.http2.Http2Settings;
 import io.netty.handler.ssl.SslCloseCompletionEvent;
+import io.netty.handler.ssl.SslHandshakeCompletionEvent;
 import io.netty.util.ReferenceCountUtil;
 import io.netty.util.concurrent.Promise;
 
@@ -284,6 +285,7 @@ public void userEventTriggered(ChannelHandlerContext ctx, Object evt) throws Exc
         }
 
         if (evt instanceof Http2ConnectionPrefaceAndSettingsFrameWrittenEvent ||
+            evt instanceof SslHandshakeCompletionEvent ||
             evt instanceof SslCloseCompletionEvent ||
             evt instanceof ChannelInputShutdownReadComplete) {
             // Expected events

core/src/main/java/com/linecorp/armeria/server/HttpServerHandler.java

@@ -89,6 +89,7 @@
 import io.netty.handler.codec.http2.Http2Settings;
 import io.netty.handler.ssl.SslCloseCompletionEvent;
 import io.netty.handler.ssl.SslHandler;
+import io.netty.handler.ssl.SslHandshakeCompletionEvent;
 
 final class HttpServerHandler extends ChannelInboundHandlerAdapter implements HttpServer {
 
@@ -643,7 +644,8 @@ public void channelReadComplete(ChannelHandlerContext ctx) throws Exception {
 
     @Override
     public void userEventTriggered(ChannelHandlerContext ctx, Object evt) throws Exception {
-        if (evt instanceof SslCloseCompletionEvent ||
+        if (evt instanceof SslHandshakeCompletionEvent ||
+            evt instanceof SslCloseCompletionEvent ||
             evt instanceof ChannelInputShutdownReadComplete) {
             // Expected events
             return;