Use backwards-compatible formats during backup

[martinmr]

This change converts the keys and posting lists to a
backwards-compatible format so that backups work accross versions of
Dgraph. The restore logic is also changed to convert the data back to
the internal Dgraph formats.

This change is 

[martinmr]

This is now ready for review.

[manishrjain]

Got a few comments.

Reviewed 1 of 3 files at r1, 1 of 1 files at r3, 1 of 1 files at r4.
Reviewable status: all files reviewed, 11 unresolved discussions (waiting on @martinmr)

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ee/backup/backup.go, line 108 at r4 (raw file):

	stream := pr.DB.NewStreamAt(pr.Request.ReadTs)
	stream.LogPrefix = "Dgraph.Backup"
	stream.KeyToList = toBackupList(pr.Request.SinceTs)

pr.toBackupList

---

ee/backup/backup.go, line 183 at r4 (raw file):

func toBackupList(since uint64) func([]byte, *badger.Iterator) (*bpb.KVList, error) {
	return func(key []byte, itr *badger.Iterator) (*bpb.KVList, error) {

No need for this.

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ee/backup/backup.go, line 186 at r4 (raw file):

		list := &bpb.KVList{}

	loop:

No need for loop.

---

ee/backup/backup.go, line 190 at r4 (raw file):

			item := itr.Item()
			if !bytes.Equal(item.Key(), key) {
				break

return?

---

ee/backup/backup.go, line 195 at r4 (raw file):

				// Ignore versions less than given timestamp, or skip older versions of
				// the given key.
				break

return

---

ee/backup/backup.go, line 250 at r4 (raw file):

				if item.DiscardEarlierVersions() || item.IsDeletedOrExpired() {
					break loop

return

---

ee/backup/restore.go, line 42 at r4 (raw file):

		dir := filepath.Join(pdir, fmt.Sprintf("p%d", groupId))
		db, err := badger.OpenManaged(badger.DefaultOptions(dir).
			WithSyncWrites(true).

Set it to false.

[martinmr]

Reviewable status: all files reviewed, 11 unresolved discussions (waiting on @golangcibot and @manishrjain)

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ee/backup/backup.go, line 180 at r1 (raw file):

Previously, golangcibot (Bot from GolangCI) wrote…

unreachable: unreachable code (from govet)

Done.

---

ee/backup/backup.go, line 200 at r2 (raw file):

Previously, golangcibot (Bot from GolangCI) wrote…

ineffectual assignment to err (from ineffassign)

Done.

---

ee/backup/backup.go, line 108 at r4 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

pr.toBackupList

Done.

---

ee/backup/backup.go, line 183 at r4 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

No need for this.

Done.

---

ee/backup/backup.go, line 186 at r4 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

No need for loop.

Done.

---

ee/backup/backup.go, line 190 at r4 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

return?

Done.

---

ee/backup/backup.go, line 195 at r4 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

return

Done.

---

ee/backup/backup.go, line 250 at r4 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

return

Done.

---

ee/backup/restore.go, line 26 at r1 (raw file):

Previously, golangcibot (Bot from GolangCI) wrote…

File is not gofmt-ed with -s (from gofmt)

Done.

---

ee/backup/restore.go, line 36 at r1 (raw file):

Previously, golangcibot (Bot from GolangCI) wrote…

func parameter backupId should be backupID (from golint)

Ignoring

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ee/backup/restore.go, line 42 at r4 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

Set it to false.

Done.