Improve keepalive ping system

Send more keepalives, record all transactions within the last minute.
We send more keepalives so that the latency calculation is more
accurate. Since we send more keepalives, we track all pending
keepalives in case multiple end up in flight.

Additionally, replace the latency calculation with a true
average over the last 5 seconds of keepalive transactions.

paper-server/patches/sources/net/minecraft/server/network/ServerCommonPacketListenerImpl.java.patch

@@ -1,6 +1,6 @@
 --- a/net/minecraft/server/network/ServerCommonPacketListenerImpl.java
 +++ b/net/minecraft/server/network/ServerCommonPacketListenerImpl.java
-@@ -29,30 +_,67 @@
+@@ -29,14 +_,14 @@
  import net.minecraft.util.profiling.Profiler;
  import org.slf4j.Logger;
  
@@ -15,11 +15,9 @@
 -    protected final Connection connection;
 +    public final Connection connection; // Paper
      private final boolean transferred;
--    private long keepAliveTime;
-+    private long keepAliveTime = Util.getMillis(); // Paper
+     private long keepAliveTime;
      private boolean keepAlivePending;
-     private long keepAliveChallenge;
-     private long closedListenerTime;
+@@ -45,14 +_,51 @@
      private boolean closed = false;
      private int latency;
      private volatile boolean suspendFlushingOnServerThread = false;