rafthttp: increase the size of streaming buffer

The old 1024 value is too small and is very likely to be full and
break the streaming in good path.

Moreover, A bigger value may help write more into the io.Writer when switching
to handle goroutine.
so say: if the network is slow for 1second, it makes sense for us to hold entries generates in 200ms. holding 1M entries makes no sense to me.