Removed section on refcounting from manual

doc/manual/api.adoc

@@ -578,95 +578,6 @@ The second `FragmentedMessage` will only marshall the part of the underlying mes
 mainly for completeness.
 
 
-[[Refcounting]]
-==== Reference-counted messages
-When a sender wants to reuse the message payload, a shared pool of payloads may be used. For example, we may want to
-create a pool of direct ByteBuffers (allocated off-heap) and wrap ByteBuffers (taken from that pool) in NioMessages.
-
-After sending an NioMessage with ByteBuffer `buf`, we may want to modify `buf` and send another NioMessage with `buf`
-as payload. However, this might fail, as the message may not yet have been sent, or it may have been dropped and
-will later get retransmitted.
-
-For example, if we set `buf` to `123`, send it, then set it to `456` and send it again, the following
-might happen:
-
-* `123` is sent as NioMessage `m1`, but `m1` is dropped
-* `456` is sent as NioMessage `m2`
-* `m1` gets retransmitted. By now, its payload is `456`, as `buf` has been changed in the meantime
-* This leads to a receiver receiving `456` twice, rather than `123` followed by `456`.
-
-Reference counting makes sure that a payload is only available to be reused when
-
-* a unicast message has been sent to a receiver and the ack from the receiver has been received by the sender,
-so that the message can be purged from the retransmission table, or
-* a multicast message (= message to all members) has been seen by all members and has been purged from
-the retransmission table.
-
-This is achieved by reference counting: a message increments a reference count (refcnt) when being sent,
-and decrements it when `JChannel.send()` returns. Retransmission protocols (such as `NAKACK2` or `UNICAST3`)
-increment refcnt, and decrement it when the message is known to have been received by the receiver(s).
-
-When refcnt becomes 0, a callback is invoked (with the message as argument), which can (e.g.) return the
-message to a pool.
-
-As example of a refcounted message is `RefcountedNioMessage`:
-
-[source,java]
-----
-public class RefcountedNioMessage extends NioMessage implements Refcountable<Message> {
-    protected final RefcountImpl<Message> impl=new RefcountImpl<>();
-
-    public RefcountedNioMessage() {
-    }
-
-    public RefcountedNioMessage(Address dest) {
-        super(dest);
-    }
-
-    public RefcountedNioMessage(Address dest, ByteBuffer buf) {
-        super(dest, buf);
-    }
-
-    public synchronized byte getRefcount() {
-        return impl.getRefcount();
-    }
-
-    @Override public synchronized RefcountedNioMessage incr() {
-        impl.incr();
-        return this;
-    }
-
-    @Override public synchronized RefcountedNioMessage decr() {
-        impl.decr(this);
-        return this;
-    }
-
-    public RefcountedNioMessage onRelease(Consumer<Message> rc) {
-        impl.onRelease(rc);
-        return this;
-    }
-}
-----
-
-It subclasses `NioMessage` and implements methods `incr()` and `decr()` of the `Refcountable` interface. The latter calls
-a function previously passed in via `onRelease()`. This function may for example return the payload of the NioMessage
-to a pool. See `RefcountedNioMessageTest` as an example.
-
-NOTE: Reference counting is an experimental feature, introduced in 5.1.0, and is WIP. The authors are interested in
-scenarios where this would be beneficial, and its performance impact. Please contact us if you are serious
-about using this feature; we're interested in feedback, suggestions and are happy to collaborate on this!
-
-===== Reference counting on the receiver side
-Reference counting is currently only implemented on the sender side.
-
-However, it could also be useful on the receiver side: a `MessageFactory` could create instances of ref-counted messages
-(the payload would be taken from a buffer pool) and increment the refcount.
-
-When such a message is received and processed by the application, the `receive(Message)` callback (or, alternatively,
-the application itself) could decrement the refcount, thus returning the buffer to the pool.
-
-
-
 [[PayloadMismatch]]
 === Mismatch between message type and getters/setters