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.
I think streaming is good to be kept as long as possible because it the fastest way to append entry. and if the term changes or connection is broken, we can find it quickly and this is not bounded by the buffer size.
let me try the value of 4096 for it in long-running tests. And see if it is good enough.