Equip SMF-Client with back pressure mechanism

[KowalczykBartek]

Equip SMF-Client with back pressure mechanism, basically, this method have to check if channel is writable,

    public void executeAsync(long methodMeta, byte[] body, final Consumer<ByteBuffer> callback) {
        int sessionId = sessionIdGenerator.next();
        LOG.info("Constructing RPC call for sessionId {}", sessionId);
        final RpcRequest rpcRequest = new RpcRequest(sessionId, methodMeta, body, callback);
        dispatcher.assignCallback(sessionId, rpcRequest.getCallback());
        //channel.isWritable() has to be checked before.
        channel.writeAndFlush(rpcRequest);
    }

otherwise, we can just kill GC, because it will not be able to keep up.