allow users to store usermetadata in lsm

[janardhan1993]

This change is 

[manishrjain]

---

Reviewed 13 of 13 files at r1.
Review status: all files reviewed at latest revision, all discussions resolved.

---

Comments from Reviewable

[srh]

Why is the user metadata one byte?

---

Review status: all files reviewed at latest revision, all discussions resolved, all commit checks successful.

---

Comments from Reviewable

[janardhan1993]

1 byte was sufficient for our use case in dgraph. I should have made it more generic, my bad.
I will change the encoding to store the length of usermetadata along with the metadata, so in case the user is not storing anything we will have only 1 byte overhead per key which should be ok.

[srh]

I'm not saying we should make it more generic. I was wondering the reason. We could always make it more generic later.

---

Review status: all files reviewed at latest revision, all discussions resolved, all commit checks successful.

---

Comments from Reviewable

[srh]

But if we did make it more generic, CAS values could use the same feature and storage overhead per CAS would be optional to the user.

---

Review status: all files reviewed at latest revision, all discussions resolved, all commit checks successful.

---

Comments from Reviewable

[srh]

Anyway, , we can always make that change separately.

---

Review status: all files reviewed at latest revision, all discussions resolved, all commit checks successful.

---

Comments from Reviewable