msgappv2 stream is used to send all MsgApp, and replaces the
functionality of msgapp stream. Compared to v1, it has several
advantanges:
1. The output message is exactly the same with the input one, which
cannot be done in v1.
2. It uses one connection to stream persistently, which prevents message
reorder and saves the time to request stream.
3. It transmits 10 addiontional bytes in the procedure of committing one
proposal, which is trivia for idle time.
4. It transmits less bytes when committing mutliple proposals or keep
committing proposals.
The messages in channel are outdated, and there is no need to send
them in the future. It also reports unreachable if there are messages
in the channel.
New rafthttp uses /raft/stream/msgapp for MsgApp stream, but v2.0 rafthttp
cannot understand it. Use the old endpoint /raft/stream instead for backward
compatibility, and plan to move to new endpoint in the version after the
next one.
