vendor: Update coreos/etcd; pick up pre-vote patch

[irfansharif]

Additionally default COCKROACH_ENABLE_PREVOTE to true. If I'm not mistaken COCKROACH_TICK_QUIESCED can be removed now.

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Changes to vendored: Needed to pick up etcd-io/etcd@a92ceee, etcd-io/etcd@93826f2 (etcd-io/etcd#8288). Part of #16950.

github.com/coreos/etcd/raft/node_test.go       |   32 +++
github.com/coreos/etcd/raft/raft.go            |   72 +++++--
github.com/coreos/etcd/raft/raft_test.go       |  160 +++++++++++++-
github.com/coreos/etcd/raft/raftpb/raft.pb.go  |   76 +++++--
github.com/coreos/etcd/raft/read_only.go       |    2 +-
github.com/coreos/etcd/raft/status.go          |   18 +-

This picks up etcd-io/etcd#7987 which we don't use as yet, so should be fine. Also picks up etcd-io/etcd#7761 which should be fine. raft/raftpb/raft.pb.go was regenerated in etcd-io/etcd#8084, which I'm less sure about, is this safe for us?

+cc @bdarnell

[cockroach-teamcity]

This change is 

[bdarnell]

etcd-io/etcd#8084 looks fine for us; the only change is to the ConfState.Unmarshal method (which looks like it's laying the groundwork for upgrading to the packed repeated int format, without actually changing anything yet).

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Review status: 0 of 4 files reviewed at latest revision, 1 unresolved discussion, all commit checks successful.

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pkg/storage/store.go, line 129 at r1 (raw file):

var enablePreVote = envutil.EnvOrDefaultBool(
	"COCKROACH_ENABLE_PREVOTE", true)

Don't do this yet. There are three settings we should flip: enable prevote, disable CheckQuorum and TickQuiesced. Let's test them with the environment variables before changing the code.

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Comments from Reviewable

[irfansharif]

Review status: 0 of 3 files reviewed at latest revision, 1 unresolved discussion.

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pkg/storage/store.go, line 129 at r1 (raw file):

Previously, bdarnell (Ben Darnell) wrote…

Don't do this yet. There are three settings we should flip: enable prevote, disable CheckQuorum and TickQuiesced. Let's test them with the environment variables before changing the code.

sure, will run before/after on chaos runs and track results on #16950. We have CheckQuorum: !enablePreVote as is.

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Comments from Reviewable