It sets 10s timeout for public GetClusterFromRemotePeers.
This helps the following cases to work well in high latency scenario:
1. proxy sync members from the cluster
2. newly-joined member sync members from the cluster
Besides 10s request timeout, the request is also controlled by dial
timeout and read connection timeout.
The request can still time out because we have set dial timeout and
read/write timeout. It increases timeout expectation from 1s to 5s,
but it makes it workable in globally-deployer cluster.
After this PR, only cluster's interface Cluster is exposed, which makes
code much cleaner. And it avoids external packages to rely on cluster
struct in the future.
1. Persist the cluster version change through raft. When the member is restarted, it can recover
the previous known decided cluster version.
2. When there is a new leader, it is forced to do a version checking immediately. This helps to
update the first cluster version fast.
Cluster version is the min major.minor of all members in
the etcd cluster. Cluster version is set to the min version
that a etcd member is compatible with when first bootstrapp.
During a rolling upgrades, the cluster version will be updated
automatically.
For example:
```
Cluster [a:1, b:1 ,c:1] -> clusterVersion 1
update a -> 2, b -> 2
after a detection
Cluster [a:2, b:2 ,c:1] -> clusterVersion 1, since c is still 1
update c -> 2
after a detection
Cluster [a:2, b:2 ,c:2] -> clusterVersion 2
```
The API/raft component can utilize clusterVersion to determine if
it can accept a client request or a raft RPC.
We choose polling rather than pushing since we want to use the same
logic for cluster version detection and (TODO) cluster version checking.
Before a member actually joins a etcd cluster, it should check the version
of the cluster. Push does not work since the other members cannot push
version info to it before it actually joins. Moreover, we do not want our
raft RPC system (which is doing the heartbeat pushing) to coordinate cluster version.
