Checking features for seed node is a bit more complicated than non-seed
node, because non-seed node can always talk to at least one seed node,
seed node may not.
In this patch, we distingush new cluster and existing cluster by
checking if the system table is empty. We relax the feature check for
new cluster because the feature check is mostly useful when upgrading an
existing cluster to prevent old node to join new cluster.
When talking to a seed node failed during the check, we fallback to the
check using features stored in the system table. This makes restarting a
seed node when no other seed node is up possible (no other seed node at
all, or other seed node is not up yet).
I tested the following scenarios.
1) start a completely new seed node in a new cluster
* system table is empty, skip the check.
2) start a cluster, restart one seed node, at least one other seed node
is up
* system table is not empty, check with shadow round, shadow round will
* succeed
3) start a cluster, restart one seed node, no other seed node is up
* system table is not empty, check with shadow round, shadow round will
* fail, fallback to system table check.
4) start a cluster, shutdown all the nodes, start one seed node with new
ip address, seed list in yaml is updated with new ip address
* system table is not empty, check with shadow round, shadow round will
* fail, fallback to system table check
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