Make use of FlushConsolidationHandler to optimize the performance of classic transport.

[He-Pin]

The migration from Netty 3 to Netty 4 will need to change the channel.write to channel.writeAndFlush ,but that can be optimized in a more performant way.

#643
asked: netty/netty#13608

Add FlushConsolidationHandler which consolidates flush operations as …
…these are expensive

Motivation:

Calling flush() and writeAndFlush(...) are expensive operations in the sense as both will produce a write(...) or writev(...) system call if there are any pending writes in the ChannelOutboundBuffer. Often we can consolidate multiple flush operations into one if currently a read loop is active for a Channel, as we can just flush when channelReadComplete is triggered. Consolidating flushes can give a huge performance win depending on how often is flush is called. The only "downside" may be a bit higher latency in the case of where only one flush is triggered by the user.

Modifications:

Add a FlushConsolidationHandler which will consolidate flushes and so improve the throughput.

Result:

Better performance (throughput). This is especially true for protocols that use some sort of PIPELINING.