Prevent nepotism with generational GCs.

Motivation:

If a single Encoder object is promoted to the old generation then every object
reachable from the promoted object will eventually be promoted as well. A queue
illustrates the problem very well. Say a sequence of inserts and deletions
generate an object graph:

   A -> B -> C -> D -> E -> F -> G -> H,

the head of the queue is E, the tail of the queue is H, and A, B, C, D are
dead. If all queue nodes are in the young generation, then a young gc will
clean up the object graph and leave us with:

   E -> F -> G -> H

on the other hand, if B and C were previously promoted to the old generation,
then a young collection assumes the refernece from C to D is from a live object
(this is a key result of generational gc, no need to mark the old generation).
Hence the young collection assumes the refence to D is a gc root and leave us
with the object graph:

   B-> C -> D -> E -> F -> G -> H.

Eventually D, E, F, G, H, and all queue nodes ever seen from this point on will
be promoted, regardless of their global live or dead status. It is generally
trivial to fix nepotism issues by simply breaking the reference chain after
dequeuing a node.

Currently Encoder objects do not null their references when removed from the
hash map. We have observed a 20X increase in promoted Encoder objects due to
nepotism.

Modifications:

Null before, after, and next fields when removing Encoder objects from maps.

Result:

Fewer promoted Encoder objects, fewer Encoder objects in the old generation,
shorter young collection times, old collections spaced further apart (nepotism
is just really bad). Enjoy.

codec-http2/src/main/java/io/netty/handler/codec/http2/hpack/Encoder.java

@@ -454,6 +454,9 @@ public final class Encoder {
         private void remove() {
             before.after = after;
             after.before = before;
+            before = null; // null references to prevent nepotism in generational GC.
+            after = null;
+            next = null;
         }
 
         /**