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.
The purpose of this buffer is:
1. non-blocking handle over data to another io routine
2. hold some pressure when the network is experiencing temp problem or there is temp high request rate. 

I do not think making the buffer much larger is a good thing. It simply delays the symptoms of a long lasting problem.