update state without proposal when leader changes in zero

[janardhan1993]

Fixes #2273

This change is 

[janardhan1993]

Review status: 0 of 3 files reviewed at latest revision, all discussions resolved.

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dgraph/cmd/zero/raft.go, line 407 at r1 (raw file):

func (n *node) triggerLeaderChange() {
	n.server.triggerLeaderChange()
	n.server.updateZeroLeader()

We can update leader information on each node without proposal. This function is called on all nodes on leader change.

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dgraph/cmd/zero/raft.go, line 408 at r1 (raw file):

	n.server.triggerLeaderChange()
	m := &intern.Member{Id: n.Id, Addr: n.RaftContext.Addr, Leader: n.AmLeader()}
	go n.proposeAndWait(context.Background(), &intern.ZeroProposal{Member: m})

Proposing it in goroutine can mess up the order and cause wrong state.

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worker/groups.go, line 250 at r1 (raw file):

	g.Lock()
	defer g.Unlock()
	if g.state != nil && g.state.Counter >= state.Counter {

We don't update state if we get any old state. Counter stores the raftindex of last state. For leader changes at zero since we don't propose, state can get updated at same counter value.

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Comments from Reviewable

[pawanrawal]

for the first two changes. Add the comments explaining them in the code please.

Not sure why we create another copy of the key. Does ReadPostingList modify the key?

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Reviewed 3 of 3 files at r1, 1 of 1 files at r2.
Review status: all files reviewed at latest revision, 3 unresolved discussions.

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Comments from Reviewable