There is no need to print log when attaching stream with the same
term because the stream is installed back immediately.
This happens a lot when etcd 2.1 connects to etcd 2.0, so we make
the change.
It is not sent out because it is useless to let remote raft step the
message.
Moreover, MsgApp stream reader can always assume that the length
of entries sent is > 0.
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.
