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The messages in channel are outdated, and there is no need to send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.
The size of MsgSnap may be very big, e.g., 1G.
If its size is big and general streaming is used to send it, it may block
the following messages for several ten seconds, which interrupts the
heartbeat heavily.
Only use pipeline to send MsgSnap.
Dial timeout is set shorter because
1. etcd is supposed to work in good environment, and the new value is long
enough
2. shorter dial timeout makes dial fail faster, which is good for
performance
