Avoid creating flush ByteBuf/Segment when not necessary.

[PokeMMO]

Currently every call to update calls flush which creates a new flush buffer and segment. Even when there is no work required to be done.

ByteBufAllocator.ioBuffer is a fairly expensive call to call every update.

Unless I am doing something wrong, Kcp#check will schedule a work duration (100ms by default) not necessarily when the next "action" is required. This might be better handled by changing Kcp#check?

[szhnet]

There are check logics both checkFlushRequired and flush method. I think it is difficult to maintain. What do you think?

[PokeMMO]

It is definitely not ideal and will cause more effort to maintain, but it is a very large performance benefit.

The other alternative is likely worse, editing flush to only create the seg and buffer when they're actually used.

https://github.com/szhnet/kcp-netty/blob/master/kcp-netty/src/main/java/io/jpower/kcp/netty/Kcp.java#L1108-L1117