rafthttp: increase the size of streaming buffer

Streaming buffer is used for:
1. hand over data to io goroutine in non-blocking way
2. hold pressure for temprorary network delay
3. be able to wait on I/O instead of data coming under high throughput

The old 1024 value is too small and is very likely to be full and
break the streaming when suffering temprorary network delay.
Merge pull request #1788 from yichengq/233

rafthttp: increase the size of streaming buffer