netty: log SocketExceptions at FINE, too

This PR changes the `NettyServerTransport#getLogLevel` method to log 
`SocketException`s to `LogLevel.FINE`, rather than exclusively pure
IOExceptions. This may fix an unintentional regression introduced in
c166ec2, although the message in my java version (14.0.1) wouldn't have
passed the old logic for quieting either. This also fixes the issue
raised in #6423 that was locked for inactivity.

This fixes 
```
[2020/05/14 20:21:52   INFO] [io.grpc.netty.NettyServerTransport.connections] Transport failed
java.net.SocketException: Connection reset
        at java.base/sun.nio.ch.SocketChannelImpl.throwConnectionReset(SocketChannelImpl.java:345)
        at java.base/sun.nio.ch.SocketChannelImpl.read(SocketChannelImpl.java:376)
        at io.netty.buffer.PooledUnsafeDirectByteBuf.setBytes(PooledUnsafeDirectByteBuf.java:288)
        at io.netty.buffer.AbstractByteBuf.writeBytes(AbstractByteBuf.java:1125)
        at io.netty.channel.socket.nio.NioSocketChannel.doReadBytes(NioSocketChannel.java:347)
        at io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:148)
        at io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:677)
        at io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:612)
        at io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:529)
        at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:491)
        at io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:905)
        at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
        at java.base/java.lang.Thread.run(Thread.java:832)
```
being logged to level INFO, which occurs whenever a socket is improperly 
closed by the client, such as with the grpc-health-probe (They've got an
[open issue](grpc-ecosystem/grpc-health-probe#34)
for this)

Author: AgentK

netty/src/main/java/io/grpc/netty/NettyServerTransport.java

@@ -38,6 +38,7 @@
 import io.netty.util.concurrent.GenericFutureListener;
 import java.io.IOException;
 import java.net.SocketAddress;
+import java.net.SocketException;
 import java.util.List;
 import java.util.concurrent.ScheduledExecutorService;
 import java.util.logging.Level;
@@ -186,6 +187,7 @@ Channel channel() {
   @VisibleForTesting
   static Level getLogLevel(Throwable t) {
     if (t.getClass().equals(IOException.class)
+        || t.getClass().equals(SocketException.class)
         || QUIET_EXCEPTIONS.contains(t.getClass().getSimpleName())) {
       return Level.FINE;
     }

netty/src/test/java/io/grpc/netty/NettyServerTransportTest.java

@@ -22,6 +22,7 @@
 import static org.junit.Assert.assertNull;
 
 import java.io.IOException;
+import java.net.SocketException;
 import java.util.logging.Level;
 import org.junit.Test;
 import org.junit.runner.RunWith;
@@ -34,6 +35,11 @@ public void unknownException() {
     assertEquals(Level.INFO, getLogLevel(new Exception()));
   }
 
+  @Test
+  public void socketException() {
+    assertEquals(Level.FINE, getLogLevel(new SocketException("Connection reset")));
+  }
+
   @Test
   public void ioException() {
     assertEquals(Level.FINE, getLogLevel(new IOException("Connection reset by peer")));