Delete tablets which don't belong

[manishrjain]

Tablet deletion:

• With recent changes, when a predicate is moved from one group to another, the tablet was left behind in the source group.
• This PR adds a way for Zero to instruct the source group to delete the predicate after successfully transferring ownership to the destination group.
• This PR also removes cleanupTablets method, so the source group does not make the decision of deleting tablets.
• Rollup operation now does the size calculation per tablet and sends it out to Zero.
• We no longer do a periodic calculateTabletSize anymore.
• When Zero receives a tablet size update, after applying the proposal, it determines which tablets need to be deleted, and sends an instruction to Alpha leader accordingly.

Potential issue: Considering the job of tablet deletion happens just after leader rollup, it is possible that the followers would still be doing their rollups when the tablet deletion happens. In that case, a race condition might happen where the deleted tablet could resurface because of roll-up operation (the key is read, then clean predicate deletes it, then the key is written back). In the future, we could cancel a roll-up operation if we determine it could cause a race condition like this.

This change is 

[martinmr]

Some comments but nothing major. Feel free to merge after looking at them.

Reviewed 5 of 5 files at r1.
Reviewable status: 4 of 5 files reviewed, 6 unresolved discussions (waiting on @manishrjain and @martinmr)

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dgraph/cmd/zero/tablet.go, line 155 at r1 (raw file):

	span.Annotate(nil, msg)

	in.DestGid = 0 // Indicates deletion for the predicate.

// Indicates deletion of predicate in the source group.

---

dgraph/cmd/zero/zero.go, line 644 at r1 (raw file):

	pl := s.Leader(gid)
	if pl == nil {
		return x.Errorf("Unable to reach leader of group: %d.", gid)

Remove period at the end.

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worker/draft.go, line 856 at r1 (raw file):

			val, _ = m.LoadOrStore(pk.Attr, sz)
		}
		sz := val.(*int64)

is sz supposed to stand for size? If so, I think renaming it to size is clearer.

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worker/draft.go, line 903 at r1 (raw file):

				pred := key.(string)
				space := val.(*int64)
				sz := atomic.LoadInt64(space)

Same here regarding sz vs size.

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worker/groups.go, line 140 at r1 (raw file):

	glog.Infof("Server is ready")

	gr.closer = y.NewCloser(3) // Match CLOSER:1 in this file.

Is this change supposed to be here?

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worker/predicate_move.go, line 182 at r1 (raw file):

	}
	if in.DestGid == 0 {
		glog.Infof("Got instructed to delete tablet: %v", in.Predicate)

"Was instructed"

[rstorr]

@CodeLingoBot capture error messages shouldn't end in a period

[codelingo]

Rule captured at https://dash.codelingo.io/repos/dgraph-io/dgraph/rules/1576

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