Fix WriteBatch APIs and Update README to include Entry API

[ashish-goswami]

This change is 

[jarifibrahim]

Looks like we do not have documentation for write batch API. We should add it in a separate PR.

Reviewed 8 of 8 files at r1.
Reviewable status: all files reviewed, 4 unresolved discussions (waiting on @ashish-goswami, @mangalaman93, and @manishrjain)

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README.md, line 219 at r1 (raw file):

err := db.Update(func(txn *badger.Txn) error {
  e := NewEntry([]byte("answer"), []byte("42"))
  err := txn.SetEntry(e)

return txn.SetEntry(e)

---

README.md, line 346 at r1 (raw file):

err := db.Update(func(txn *badger.Txn) error {
  e := NewEntry([]byte("answer"), []byte("42")).WithTTL(time.Hour)
  err := txn.SetEntry(e)

return txn.SetEntry(e)

---

README.md, line 359 at r1 (raw file):

err := db.Update(func(txn *badger.Txn) error {
  e := NewEntry([]byte("answer"), []byte("42")).WithMeta(byte(0))
  err := txn.SetEntry(e)

return txn.SetEntry(e)

---

README.md, line 365 at r1 (raw file):

`Entry` APIs can be used to add the user metadata and TTL for same key. This `Entry`
then can be set using `Txn.SetEntry()`.

Can we add an example of withTTL and withMeta used together?

[manishrjain]

Defer to @jarifibrahim for final review. I got a comment.

Reviewed 8 of 8 files at r1.
Reviewable status: all files reviewed, 5 unresolved discussions (waiting on @ashish-goswami and @mangalaman93)

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README.md, line 358 at r1 (raw file):

```go
err := db.Update(func(txn *badger.Txn) error {
  e := NewEntry([]byte("answer"), []byte("42")).WithMeta(byte(0))

Use byte(1) or 0xab or something to show meta being set. A zero meta is equivalent of not setting one.

[ashish-goswami]

Reviewable status: 7 of 8 files reviewed, 5 unresolved discussions (waiting on @jarifibrahim, @mangalaman93, and @manishrjain)

---

README.md, line 219 at r1 (raw file):

Previously, jarifibrahim (Ibrahim Jarif) wrote…

return txn.SetEntry(e)

I just wanted to have it simple here, hence wrote it in two lines.

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README.md, line 346 at r1 (raw file):

Previously, jarifibrahim (Ibrahim Jarif) wrote…

return txn.SetEntry(e)

same reason.

---

README.md, line 358 at r1 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

Use byte(1) or 0xab or something to show meta being set. A zero meta is equivalent of not setting one.

Done.

---

README.md, line 359 at r1 (raw file):

Previously, jarifibrahim (Ibrahim Jarif) wrote…

return txn.SetEntry(e)

same reason.

---

README.md, line 365 at r1 (raw file):

Previously, jarifibrahim (Ibrahim Jarif) wrote…

Can we add an example of withTTL and withMeta used together?

Done.

[jarifibrahim]

Reviewed 1 of 1 files at r2.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @mangalaman93 and @manishrjain)

[manishrjain]

Reviewed 1 of 1 files at r2.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @mangalaman93 and @manishrjain)